Render docs with features enabled

[Mark-Simulacrum]

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(...)) (rust-lang/rust#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

[Mark-Simulacrum]

cc aws/s2n-quic#1564, which is a related issue for s2n-quic