feat: add support for aws-lc-rs alongside ring

[sshockwave]

Closes #93
Part of webrtc-rs/webrtc#801

The roadmap listed post-quantum support in the "Future Exploration (Beyond 2027)" section. However, I think support for aws-lc-rs should gain more priority not because quantum computer is going to be realized anytime soon, but because the ecosystem (e.g. reqwest, rustls) is migrating towards aws-lc-rs by default. Requiring downstream crates to include two crypto providers is not a good idea. There were also discussions that ring was probably not intended for production use.

Crate rtc-ice and rtc-turn are also affected by the feature because they depend on rtc-stun, but I'm not sure I should add feature gates just to enable/disable the feature of a dependency. It seems recommended here but it is verbose and we seldom use these crates on their own anyway.

Test commands:

# `rtc` must be tested separately since interop tests needs `ring` dependency,
# which will clash with `aws-lc-rs` unless the `__testing` helper chooses a provider for `rustls`.
cargo test -p rtc --no-default-features --features aws-lc-rs,__testing

# I'm hesitant about adding a feature gate to the examples so it's still using the default `ring`
cargo test --workspace --exclude rtc --exclude rtc-examples --no-default-features --features aws-lc-rs

# The original test runs fine
cargo test --workspace

# No dependency leaks
cargo tree -e no-dev -i aws-lc-rs # expect an error
cargo tree -e no-dev -i ring --no-default-features --features aws-lc-rs

# Dependency listing works normally
cargo tree -e no-dev -i ring
cargo tree -e no-dev -i aws-lc-rs --no-default-features --features aws-lc-rs

By the way, will a similar PR to v0.17.x be accepted?

[sshockwave]

Another way we could deal with the testing --all-targets failure is to use one of the providers when both features are enabled. I'd like to generate some warnings in this case but currently there's no direct way to do so in rust.