feat: token-based authentication

[edwinkys]

Purpose

This PR adds a layer of HTTP authentication before performing actions to the database via the endpoints. This authentication is applied only for /index and /values routes.

Approach

This PR add a feature that will get x-oasysdb-token header from the request and match it against the OASYSDB_TOKEN environment variable used to run the server. If the token doesn't match, the server will return a 401 unauthorized response.

Testing

• I have tested this PR locally.
• I added tests to cover my changes, if not applicable, I have added a reason why.

I tested this PR by running cargo test and manually checking functionality by calling the endpoints directly from Insomnia. There is no new tests being added to the test suite. But, I added functionality to the existing tests to guard for unauthorized request without or invalid x-oasysdb-token.

Chore checklist

• I have updated the documentation accordingly.
• I have added comments to most of my code, particularly in hard-to-understand areas.