Render docs with features enabled

Problem:

docs.rs doesn't show the available functionality gated behind features (e.g. QUIC methods).

Solution:

We should leverage the docs.rs metadata (docs: https://docs.rs/about/metadata) to enable relevant features in the public documentation. Ideally, the cfg'd methods would also use doc(cfg(...)) (https://github.com/rust-lang/rust/issues/43781), so that the rendered docs make clear in a readable fashion what features need to be enabled. That feature is still unstable but docs.rs uses nightly compilers so that's not a problem.

Does this change what S2N sends over the wire? No
Does this change any public APIs? No
Which versions of TLS will this impact? n/a

aws / s2n-tls

Render docs with features enabled #4641

Problem:

Solution: