Add proof of concept for the `changes` endpoint

[EshaanAgg]

This PR fixes part of #15.

I have added the changes.rs file, which implements one method to get all the changes for now. I first wanted to use this PR as a proof of concept and get confirmation, after which I would add more endpoints.

The primary concerns I had were:

• I used Option to represent the optional query parameters and created a custom utility function called generate_query_url to get the query string from multiple optional parameters. Does this implementation look good? Should we consider shifting this function to a more "global" crate?
• How do I test if the code I added is working? I couldn't see mock servers or integration test infrastructure in the code base.

[unicodeveloper]

Hey @kymppi Palease can you help review this PR?

[EshaanAgg]

@unicodeveloper @kymppi Can you please add the hacktoberfest tag to this repository or the hacktoberfest-accepted label to this PR?

[unicodeveloper]

@unicodeveloper @kymppi Can you please add the hacktoberfest tag to this repository or the hacktoberfest-accepted label to this PR?

That was added yesterday.

[EshaanAgg]

@unicodeveloper I can't see the same. It must either be the hacktoberfest tag on the repository or the hacktoberfest-accepted tag on a PR. I can only see the hacktoberfest tag on my PR, causing the same to be excluded from the count of Hacktoberfest.

Also when will this PR be merged? I would love to add additional functionality to this repository once this base PR is merged.

[EshaanAgg]

@unicodeveloper Am I missing something? Is there any additional step in getting the PR merged?

[unicodeveloper]

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