feat(http1): add support for receiving trailer fields

[hjr3]

Closes #2703

In https://github.com/hyperium/hyper/pull/3375 we were strict about sending trailers per RFC 7230. In this PR, I have been much more accepting of trailers. Is this the behavior we want?

Some questions:

• If the client does not send TE: trailers but the server sends trailer headers, should trailers be parsed and passed along?
• If the server does not include a trailers: ... header indicating which trailer headers are being returned, should trailers be parsed and passed along? I think this one is almost certainly "yes".
• If the server includes trailers with invalid header names, such as Content-Length: 5, should that still be parsed and passed along?

[hjr3]

@seanmonstar Please feel free to assign me any relevant issues. I already some blog posts on sending trailers:

• https://hermanradtke.com/streaming-body-trailing-headers-axum-0-6/
• https://hermanradtke.com/streaming-body-trailing-headers-hyper-1-0-and-axum-0-7/

I had planned on writing one for receiving trailers. I had a note to ask you if you wanted this sort of content in examples, the hyper.rs guides, etc.