[PSM Interop] Simplify PSM Interop Kokoro buildscripts (v1.62.x backport) (#36533)

Backport of #36450 to v1.62.x.
---
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](https://github.com/grpc/psm-interop/blob/main/.kokoro/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 grpc/psm-interop#73.
- Additional fixes to the shared lib: grpc/psm-interop#78,
grpc/psm-interop#79

Author: sergiitk

tools/internal_ci/linux/grpc_xds_k8s_lb.cfg

@@ -15,7 +15,7 @@
 # Config file for the internal CI (in protobuf text format)
 
 # Location of the continuous shell script in repository.
-build_file: "grpc/tools/internal_ci/linux/grpc_xds_k8s_lb.sh"
+build_file: "grpc/tools/internal_ci/linux/psm-interop-test-cpp.sh"
 timeout_mins: 240
 action {
   define_artifacts {
@@ -24,3 +24,7 @@ action {
     strip_prefix: "artifacts"
   }
 }
+env_vars {
+  key: "PSM_TEST_SUITE"
+  value: "lb"
+}

tools/internal_ci/linux/grpc_xds_k8s_lb.sh

@@ -15,7 +15,7 @@
 # Config file for the internal CI (in protobuf text format)
 
 # Location of the continuous shell script in repository.
-build_file: "grpc/tools/internal_ci/linux/grpc_xds_k8s_lb_python.sh"
+build_file: "grpc/tools/internal_ci/linux/psm-interop-test-python.sh"
 timeout_mins: 180
 action {
   define_artifacts {
@@ -24,3 +24,7 @@ action {
     strip_prefix: "artifacts"
   }
 }
+env_vars {
+  key: "PSM_TEST_SUITE"
+  value: "lb"
+}