Add `extmap-allow-mixed` SDP attribute #1876

Job	Run time
lint	41s
test (ubuntu-latest, stable)	1m 39s
cargo-deny (aarch64-apple-ios)	18s
test (ubuntu-latest, beta)	1m 41s
cargo-deny (aarch64-linux-android)	18s
test (ubuntu-latest, 1.65.0)	2m 10s
cargo-deny (i686-pc-windows-gnu)	19s
test (macos-latest, stable)	59s
cargo-deny (i686-pc-windows-msvc)	20s
test (macos-latest, beta)	1m 3s
cargo-deny (i686-unknown-linux-gnu)	23s
test (macos-latest, 1.65.0)	1m 23s
cargo-deny (wasm32-unknown-unknown)	18s
test (windows-latest, stable)	10m 4s
cargo-deny (x86_64-apple-darwin)	17s
test (windows-latest, beta)	9m 7s
cargo-deny (x86_64-apple-ios)	17s
test (windows-latest, 1.65.0)	10m 33s
cargo-deny (x86_64-pc-windows-gnu)	18s
cargo-deny (x86_64-pc-windows-msvc)	17s
cargo-deny (x86_64-unknown-linux-gnu)	21s
cargo-deny (x86_64-unknown-redox)	20s
	43m 6s

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