Adding `FirebaseToken` guard

[rgarlik]

This PR adds a FirebaseToken guard that greatly improves the user experience when implementing route handlers by eliminating the need of manually checking token validity. Simply including the FirebaseToken guard rejects traffic that lacks an auth header or that includes an invalid Firebase JWT. This PR solves #4.

Example usage:

#[macro_use]
extern crate rocket;
use rocket_firebase_auth::{FirebaseAuth, FirebaseToken};

#[launch]
async fn rocket() -> _ {
    let firebase_auth = FirebaseAuth::builder()
        .json_file("firebase_creds.json") // make sure this file exists
        .build()
        .unwrap();

    rocket::build()
        .manage(firebase_auth) // Add FirebaseAuth as a managed service
        .mount("/", routes![handler])
}

#[get("/")]
async fn handler(guard: FirebaseToken) -> String {
    // Including the FirebaseToken guard is enough
    // the handler will run only if the token is valid.
    // The request guard won't work if FirebaseAuth state is not present. 
    format!("Hello, you're logged in as user ID {}", guard.token.uid)
}

This should not break compatibility with old implementations of the library (manual token checking in handler function body). I also think this should be the preferred way of using the library.

A full list of changes:

• Adding the FirebaseToken guard
• Adding a new, very simple usage example
• Updating README to contain the new approach
• Updating README to contain a "Quickstart" example at the beginning (I think this will help drive more people to use the library)
• Updating README and pushing the "upgrading from v2 to v3" section further to the bottom as I feel it's not very important to include that high up

Excited to hear your thoughts and feedback.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +3.92 :tada:

Comparison is base (4aef1b7) 51.83% compared to head (711733e) 55.76%.

:exclamation: Current head 711733e differs from pull request most recent head d248474. Consider uploading reports for the commit d248474 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5 +/- ## ========================================== + Coverage 51.83% 55.76% +3.92% ========================================== Files 5 6 +1 Lines 218 269 +51 ========================================== + Hits 113 150 +37 - Misses 105 119 +14 ``` | [Impacted Files](https://codecov.io/gh/DrPoppyseed/rocket-firebase-auth/pull/5?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada) | Coverage Δ | | |---|---|---| | [src/errors.rs](https://codecov.io/gh/DrPoppyseed/rocket-firebase-auth/pull/5?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada#diff-c3JjL2Vycm9ycy5ycw==) | `19.35% <ø> (ø)` | | | [src/firebase\_token.rs](https://codecov.io/gh/DrPoppyseed/rocket-firebase-auth/pull/5?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada#diff-c3JjL2ZpcmViYXNlX3Rva2VuLnJz) | `0.00% <0.00%> (ø)` | | | [src/lib.rs](https://codecov.io/gh/DrPoppyseed/rocket-firebase-auth/pull/5?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada#diff-c3JjL2xpYi5ycw==) | `77.63% <ø> (+9.21%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://codecov.io/gh/DrPoppyseed/rocket-firebase-auth/pull/5/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Haruki+Jay+Shimada)

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[peaske7]

@rgarlik First off, thank you so much for publishing this PR! The overall idea is great and is a welcome change! Before we merge this, could you add some tests to check against the happy and unhappy paths? Also, if you run cargo fmt before publishing commits, cargo will automatically format the code to be aligned with the style specified in the .rustfmt.toml file.

[rgarlik]

Hey @DrPoppyseed, I apologize, I had a lot of stuff going on. I've pushed all of the changes you were talking about. You can go ahead and check it out.

[rgarlik]

By the way, I had a job interview the other day and the interviewer saw this PR and asked a bit about it, I think it kinda helped me because he doesn't see a lot of people do Rust stuff. I didn't even know he'd see it lol.

[rgarlik]

added the changes you wanted, only thing left is the one where I left the discussion open

[peaske7]

Sorry it took so long! Merging it in!