libostree: Add support for ETag and Last-Modified headers

libostree: Add support for ETag and Last-Modified headers

Add support in the soup and curl fetchers to send the `If-None-Match`
and `If-Modified-Since` request headers, and pass on the `ETag` and
`Last-Modified` response headers.

This currently introduces no functional changes, but once call sites
provide the appropriate integration, this will allow HTTP caching to
happen with requests (typically with metadata requests, where the data
is not immutable due to being content-addressed). That should reduce
bandwidth requirements.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>