Support Conditional Requests

[TheAlan404]

Is your suggested enhancement related to a problem? Please describe.

GitHub API has a feature called Conditional Requests which allows clients to cache the ETag of requests to ask if the content has changed yet.

Describe the solution you'd like

Probably using the actix_middleware_etag crate this can be implemented into Labrinth.

Implementing conditional requests, if clients implement properly, should use less bandwidth and a small amount of less response times, im guessing

Describe alternatives you've considered

None

Additional context

No response