Feature: Add --auth Functionality

[biodrone]

Hi folks!

Started using this action and found the need for quick initial root user creation, also saw an open issue that didn't appear to have been solved so I thought I'd sort out a solution.

This PR:

• Adds parsing of the MONGO_INITDB_ROOT_USERNAME and MONGO_INITDB_ROOT_PASSWORD env vars (as mongodb-username and mongodb-password inputs respectively), which automatically enable --auth as per the official docker container logs;
• Adds mongodb-db which will create a DB;
• Modifies the single instance test to use these auth parameters;
• Updates the README to document the new options (including a note as to how this apparently doesn't work in conjunction with --replSet)

Hopefully fixes #12

Also, sidenote: I've tested the vars as blank strings and the container just ignores them, I almost added them to the test matrices but it wound up making like 86 jobs per test because of al the node and JS versions being tested :shrug:

[marcuspoehls]

@biodrone Hey Josh, thank you for going ahead and creating the pull request 🙌

I'll look through it on Monday and report back.

One question: you changed all tests and GitHub Actions workflow files to use the three new variables (database, user, password). Are the fields now required to be defined?

[biodrone]

No, it's fine if they're left blank. The only caveat is that for user/pass you have to use both or neither, one and not the other won't activate --auth (underlying container behaviour 🤷).

I did some tests with blank variables for those in the matrix but because it then becomes 3x3x2x2x2 tests it creates like 85 tests 😅 Recommend trying it once to make sure you're happy with the new behaviour but probably not something to leave on long term...

[marcuspoehls]

@biodrone Thank you for the pull request Josh! I’ll release a new version of this action tomorrow 👍

[biodrone]

Awesome, you're welcome @marcuspoehls ! Looking forward to using it :smile: