Implement `RTCRtpSender.getCapabilities` method #1594

Job	Run time
Run wasm-bindgen crate tests (unix)	15m 5s
Clippy	4m 53s
Check	58s
Verify that web-sys is compiled correctly	49s
Rustfmt	7s
Run js-sys crate tests	2m 4s
Run native tests	1m 47s
Run wasm-bindgen crate tests with multithreading enabled	1m 29s
Run web-sys crate tests	6m 3s
Run wasm-bindgen-webidl crate tests	2m 2s
build_benchmarks	52s
build_examples	3m 26s
build_nightly	1m 46s
dist_linux_aarch64_gnu	2m 52s
dist_linux_x86_64_musl	2m 48s
dist_macos_aarch64	2m 19s
dist_macos_x86_64	3m 33s
dist_windows	3m 48s
Build and test the deno example	43s
Test TypeScript output of wasm-bindgen	1m 16s
Run UI compile-fail tests	1m 12s
doc_api	1m 6s
doc_book	14s
test_examples	2m 10s
deploy	1m 7s
	1h 4m 29s

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