Add `opt_set` methods mirroring `av_opt_set` #365

Job	Run time
rust_clippy_check_ubuntu	5m 9s
build_static_and_test_ubuntu (release/6.0, nightly, no valgrind)	5m 21s
build_dynamic_and_test_windows_pre	6m 45s
build_static_and_test_ubuntu (release/6.0, 1.70.0, no valgrind)	5m 26s
rust_clippy_check_windows	2m 8s
docs_rs_check	1m 13s
build_static_and_test_ubuntu (release/6.1, nightly, valgrind)	1h 2m 48s
rustfmt_check (macos-latest)	20s
build_static_and_test_ubuntu (release/6.1, nightly, no valgrind)	6m 2s
rustfmt_check (windows-latest)	34s
build_static_and_test_ubuntu (release/6.1, 1.70.0, no valgrind)	5m 20s
rustfmt_check (ubuntu-latest)	15s
build_dynamic_and_test_windows (nightly)	2m 32s
build_dynamic_and_test_windows (1.70.0)	2m 33s
	1h 46m 26s

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