Implement `RTCRtpSender.getCapabilities` method #1596

Job	Run time
Clippy	5m 34s
Run wasm-bindgen crate tests (unix)	5m 33s
Check	1m 4s
Verify that web-sys is compiled correctly	53s
Run js-sys crate tests	2m 14s
Rustfmt	14s
Run native tests	3m 5s
Run wasm-bindgen crate tests with multithreading enabled	1m 36s
Run web-sys crate tests	6m 13s
build_benchmarks	1m 0s
build_examples	3m 28s
build_nightly	1m 45s
dist_linux_x86_64_musl	2m 29s
dist_linux_aarch64_gnu	2m 51s
dist_macos_x86_64	4m 12s
Build and test the deno example	57s
Run UI compile-fail tests	1m 18s
Test TypeScript output of wasm-bindgen	1m 21s
Run wasm-bindgen-webidl crate tests	2m 12s
dist_macos_aarch64	4m 21s
dist_windows	2m 54s
doc_api	1m 16s
doc_book	37s
test_examples	2m 31s
deploy	1m 4s
	1h 0m 42s

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