### Changes in this PR
* Refactor and remove some Core/C++ dependencies to simplify Python Observability package build process.
* Refactored code to read config at Python layer.
* Enable observability build from source.
* Add observability to run_test.
* Currently it's only enabled in Linux.
* Add error handler in run_test loaders.
* Current framework will always visit modules in test directory then decide which tests to skip.
* Since we're not building Observability for MacOS and Windows this step will fail with error `No module named 'grpc_observability'`.
* After the change we'll just skip those modules.
* We still have `_sanity_test` to make sure all tests are loaded correctly for each platform.
* Remov OC dependency as we're migrating to OTel.
* Also removed trace from testing.
* Note that trace propagation function was also removed because of this.
### Testing
* Passed existing tests.
* Tested locally, able to build observability from source using `GRPC_PYTHON_BUILD_WITH_CYTHON=1 pip install .`.
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Closes#34207
PiperOrigin-RevId: 590258014
Stanley Cheung