[All] improve APIs of all crates

[rainliu]

Follows the following documents to improve APIs of webrtc-rs crates to make them more idiomatic rust:

Rust API Guidelines: https://rust-lang.github.io/api-guidelines/about.html Rust Design Patterns: https://rust-unofficial.github.io/patterns/intro.html Elegant Library APIs in Rust: https://deterministic.space/elegant-apis-in-rust.html