Handle requests according to [RFC 9110](https://httpwg.org/specs/rfc9110.html).
Cross check against https://docs.google.com/spreadsheets/d/1GzJAnv1g16ERDpLONmdFlkwsSIxc5Dw9w4xKCtUV_ho/edit#gid=20371120
Spec parameter descriptions
* `If-None-Match`: Conditionally request a resource only if it has been modified since the specified entity revision numbers
* `If-Match`: Conditionally request a resource only if it has not been modified since one of the specified entity revision numbers
* `If-Modified-Since`: Conditionally request a resource only if it has been modified after the specified date
* `If-Unmodified-Since`: Conditionally request a resource only if it has not been modified after the specified date
Note:
* existing functionality is mostly correct except for the POST endpoint as seen in the spreadsheet
* If-Match and If-Unmodified-Since on GET endpoints may also be incorrect
* All combinations of conditional header and HTTP method to be covered in the specification and autodocs (i.e. even the less clear use cases)