Add support for custom user-managed data using `AVIOContextOpaque` #584

Job	Run time
rust_clippy_check_windows	1m 30s
rust_clippy_check_ubuntu	6m 47s
build_dynamic_and_test_windows_pre	7m 20s
rustfmt_check (windows-latest)	32s
rustfmt_check (ubuntu-latest)	17s
rustfmt_check (macos-latest)	19s
build_static_and_test_ubuntu (release/6.0, nightly, no valgrind)	7m 19s
build_static_and_test_ubuntu (release/7.0, nightly, no valgrind)	7m 22s
build_static_and_test_ubuntu (release/6.1, 1.81.0, no valgrind)	6m 25s
build_static_and_test_ubuntu (release/6.0, 1.81.0, no valgrind)	7m 42s
build_static_and_test_ubuntu (release/7.0, 1.81.0, no valgrind)	6m 43s
build_static_and_test_ubuntu (release/7.1, 1.81.0, valgrind)	7m 49s
build_static_and_test_ubuntu (release/6.1, nightly, no valgrind)	6m 19s
docs_rs_check	1m 21s
build_static_and_test_ubuntu (release/7.1, nightly, no valgrind)	10m 3s
build_static_and_test_ubuntu (release/7.0, 1.81.0, valgrind)	6m 56s
build_static_and_test_ubuntu (release/7.1, 1.81.0, no valgrind)	6m 27s
build_dynamic_and_test_windows (nightly)	1m 55s
build_dynamic_and_test_windows (1.81.0)	1m 48s
	1h 34m 54s

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