Draft: plugins: Improve footprint by allowing element selection during static link stage

This MR intends to offer a way to create a very light gstreamer-full library containing only a set of elements from a given plugin. During the link stage of gstreamer-full, the dead code path will be removed and the final binary size will be smaller.

This MR depends on another MR pending on gst-build repository:

gst-build!176 (closed)

To give a little bit of measurement:

build command: MESON build_static_minimal -Ddefault_library=static -Dlibav=disabled -Dpython=disabled -Dlibnice=disabled -Ddevtools=disabled -Dges=disabled -Drtsp_server=disabled -Dgst-examples=disabled -Dtests=disabled -Dexamples=disabled -Ddoc=disabled -Dbad=disabled -Dugly=disabled -Dgood=disabled -Dbase=disabled -Dbuildtype=release -Dbackend=ninja

• With this MR and filesrc, fakesink enabled: 1.7M
• Without this MR: 2.3M

lib (stripped)	1	2	3	4	5	6
ligstreamer-full.so	2337112	2341176	2250776	1793240	1641400	1629080

1. before any mods in gst repos (except removing base and good from the build, see commit): build without -Dgstreamer:optimize-gc-sections=true -Dgst-full-elements=coreelements:filesrc,fakesink,identity in the meson command line.
2. without element separation: build without -Dgstreamer:optimize-gc-sections=true -Dgst-full-elements=coreelements:filesrc,fakesink,identity in the meson command line.
3. with element separation: build without -Dgstreamer:optimize-gc-sections=true
4. Final build with command line above.
5. With commit 9351982e(split init in each element file) without garbage collection
6. With commit 9351982e with garbage collection