Add options for all plugins, add a new target + option for examples, check for tomllib

meson: Require tomllib / tomli python modules explicitly

These are required by dependencies.py

cargo_wrapper: Fix setting of PKG_CONFIG_PATH and CARGO_TARGET_DIR

Don't need to use an env var for the latter.

meson: Add options for all plugins

Required a slight rework of the build file, and how options are passed
to cargo_wrapper.py

Necessitated a bump of the required gstreamer version to 1.20, which
should be fine for the meson build since its primary function is to be
built as part of the gstreamer monorepo build to get a dev env.

meson: Add an option to build examples

Required renaming threadshare/benchmark to threadshare/ts-benchmark
because 'benchmark' as a target name is reserved for meson's
`benchmark` target.

Disabled by default because cargo decides that it has to rebuild
everything, and is really slow because of that.

Also required adding --features for setting features required by the
examples.