Remove C# implementation (individual packages will continue to be maintained through v2.46.x patches or moved to grpc-dotnet) (#29225)

* remove examples/csharp

* remove reference to examples/csharp

* remove C# references from doc/

* remove src/csharp

* update test_creds locations

* remove C# distribtests

* remove buildgen templates for files under src/csharp

* remove grpc_csharp_ext C build target

* regenerate projects

* update tools/run_tests/README.md

* change many C# test/build scripts to noop

* unbreak run_tests.py -l csharp

* unbreak C# build_package targets

* unbreak C# distribtests

* remove C# pre-build steps from prepare_build_windows.bat

* remove src/csharp from clang_format

* remove csharp and csharpcoreclr from run_interop_tests.py

* remove C# interop test docker images

* Revert "remove src/csharp"

This reverts commit 97f2b6d707cc1140788b3618f9ab3de36a3dc2fb.

* remove src/csharp, but keep Grpc.Tools

* add back templates/src/csharp/build/dependencies.props.template

* remove everything but Grpc.Tools and Grpc.Tools.Tests from sln

* Revert "unbreak run_tests.py -l csharp"

This reverts commit 529b5e2ec19532038b41e935ea9f4f015ac60881.

* revive Grpc.Tools.Tests through run_tests.py

* Revert "unbreak C# build_package targets"

This reverts commit 79d8d83c8bfacdca5c41fa4b33a97751167c7553.

* experiment with build_nuget.sh fixes

* simplify create_fake_native_artifacts.sh

* building of Grpc.Tools in package_targets.py

* move shared files from Grpc.Core to Grpc.Tools

* yapf format: distribtest_targets.py

* Revert "remove C# pre-build steps from prepare_build_windows.bat"

This reverts commit bd38956a2b3a670fc51edb0de08aed48e9c885b6.

* update C# README.md

* disable C# in gke OSS benchmarks