Add Entity Tags or Last-Modified to Collection Resources to avoid unnecessary data transfers

[KrausMatthias]

Is your feature request related to a problem? Please describe.
Currently repeated requests to the same resource lead to a re-transfer of the resource even if it didn't change.

Describe the solution you'd like
Endpoints should include ETag or Last-Modified Headers in their responses to allow client side caching and avoid re-transfers
as specified e.g. in https://docs.ogc.org/is/19-072/19-072.html#_1ff2bb3e-6b42-4216-843f-4aada3305102

Conditional requests including a If-Modified-Since or a If-None-Match Header should be answered with Status 304 if the resource wasn't modified since the last request.

This might also help with issues as in #2141

[C-Loftus]

Not sure if this helps but my colleague was working on this , #1998 , for the want-content-digest header. Not sure the status of the PR into upstream but it exists in our fork at least

I know this is a bit orthogonal to If-Modified-Since but wanted to call it out. Might be interesting or worthwhile to combine these in some way.