Add gun:trailers/3 for HTTP/1.1 and HTTP/2

[the-mikedavis]

This is a work-in-progress for adding gun:trailers/3. So far I adapted what data/7 is doing in HTTP/1 and HTTP/2 - I still need to look at adding test cases.

Closes #356

[essen]

This will need to be made to work with HTTP/3 as well. It likely already works if you do the same as HTTP/2. Tests will confirm. Worst case, using trailers with HTTP/3 should trigger an error (there's plenty of things not implemented with HTTP/3 so if tests show it doesn't work it's fine to error out for now).

[the-mikedavis]

For HTTP/3, a case that errors is probably the easiest (for me at least 😅) for now. It looks like we need to access/update the #http3_state.http3_machine for the QPACK encoding. (I'm pretty familiar with HTTP/2 but not HTTP/3 - maybe this is easier than I am thinking.) gun_http3:data/7 uses cow_http3:data/1 but I think we want to use cow_http3:headers/1 on the headers block from cow_http3_machine:prepare_trailers/3 instead.

[essen]

Right if it's not the same as HTTP/2 let's error out for now in gun_http3.