Add `opt_set` methods mirroring `av_opt_set` #368

Job	Run time
rust_clippy_check_ubuntu	4m 51s
build_dynamic_and_test_windows_pre	5m 32s
docs_rs_check	49s
build_static_and_test_ubuntu (release/6.0, nightly, no valgrind)	5m 20s
rust_clippy_check_windows	2m 27s
build_static_and_test_ubuntu (release/6.0, 1.70.0, no valgrind)	5m 23s
rustfmt_check (macos-latest)	20s
build_static_and_test_ubuntu (release/6.1, nightly, valgrind)	1h 3m 36s
rustfmt_check (windows-latest)	35s
rustfmt_check (ubuntu-latest)	15s
build_static_and_test_ubuntu (release/6.1, nightly, no valgrind)	5m 58s
build_static_and_test_ubuntu (release/6.1, 1.70.0, no valgrind)	5m 40s
build_dynamic_and_test_windows (nightly)	2m 33s
build_dynamic_and_test_windows (1.70.0)	2m 24s
	1h 45m 43s

Provide feedback