PSM Interop: simplify Kokoro buildscripts (@grpc/[email protected] backport)

[sergiitk]

Backport of #2729 to @grpc/[email protected].

Integrates the new features of the the Kokoro PSM Interop install library introduced in grpc/psm-interop#73.

Nearly all common functionality was moved from per-language/per-branch PSM Interop build scripts to psm_interop_kokoro_lib.sh:

1. The list of tests in the each test suite
2. Per-test-suite flag customization
3. run_test methods
4. build_docker_images_if_needed methods
5. Generic build_test_app_docker_images methods (simple docker build + docker push + docker tag). grpc-java is one exception, as it doesn't run docker directly, but a cloudbuild flow.

Now all PSM Interop jobs share the same buildscripts by all test suites:

1. buildscript that invokes the test: psm-interop-test-{language}.sh (configured as build_file in the build cfg)
2. buildscript that builds the xDS test client/server and publishes them as a Docker image: psm-interop-build-{language}.sh (conventional name called from psm_interop_kokoro_lib.sh)

psm-interop-test-{language}.sh:

1. Sets GRPC_LANGUAGE, BUILD_SCRIPT_DIR environment variables.
2. Downloads the shared psm_interop_kokoro_lib.sh from the main branch of the psm-interop repo.
3. Sources psm-interop-build-{language}.sh
4. Calls psm::run "${PSM_TEST_SUITE}" (PSM_TEST_SUITE configured in the cfg file).

psm-interop-build-{language}.sh:

1. Defines psm::lang::build_docker_images which is called from psm_interop_kokoro_lib.sh.
2. Invokes any repo-specific logic.
3. May use psm::build::docker_images_generic for generic Docker build, tag, push, or provide implement its own build/publish method.

References:

• b/288578634
• See the full list of the new features at Move all common buildscript functionality to psm_interop_kokoro_lib.sh psm-interop#73.
• Additional fixes to the shared lib: kokoro: Call 'docker push' correctly while pushing images psm-interop#78, Shared kokoro lib: address feedback psm-interop#79