Auto merge of #29781 - mrobinson:bootstrap-gstreamer, r=mukilan

Implement `bootstrap-gstreamer` for all platforms

This change makes it so that the Platform classes can now handle installing GStreamer dependencies and properly setting up the environment including when cross-compiling. For Windows and Linux, prepackaged GStreamer is now installed into `target/dependencies/gstreamer` when not installed system-wide. In addition this change:

1. The Environment path append helper is moved to `util.py` and a new `prepend` version is added.
2. `set_run_env` and `build_dev` functions are combined and include more code from callers so environment setup is more often in the same place. Now code that used to call `set_run_env` calls `build_dev` and then `os.environ.update(...)`. We can further refine this with the `is_build` argument later.
4. Python typing information is added in many places.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #25335
- [x] These changes do not require tests because they are to the build scripts.

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.github/workflows/mac.yml

@@ -66,7 +66,7 @@ jobs:
       - name: Bootstrap
         run: |
           python3 -m pip install --upgrade pip virtualenv
-          bash etc/install_macos_gstreamer.sh
+          python3 ./mach bootstrap-gstreamer
           brew install gnu-tar
       - name: Release build
         run: |