Move all common buildscript functionality to psm_interop_kokoro_lib.sh

[sergiitk]

Major rework of the shared Kokoro PSM Interop install library.

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.

Added new options to control PSM interop test driver execution via environment variables:

# Overrides the list of test to run. A whitespace-separated string. Example:
# PSM_TESTS="app_net_test baseline_test"
readonly PSM_TESTS="${PSM_TESTS:-}"

# A space-separated string with extra flags to append to the test driver arguments.
# Can be used to execute a run and test a new flag value, f.e.:
# PSM_EXTRA_FLAGS="--noenable_workload_identity --td_bootstrap_image=us-docker.pkg.dev/new-image..."
# In addition, can be used to run a single test in a suite. 
# F.e. to run only test_mtls in security_test:
# PSM_TESTS="security_test" PSM_EXTRA_FLAGS="SecurityTest.test_mtls"
readonly PSM_EXTRA_FLAGS="${PSM_EXTRA_FLAGS:-}"

Improved logging:

1. Buildscript test logs are now prepended with the date:
   • when a shell command passed to psm::tools::run_verbose
   • psm::tools::log is used in place of echo
2. Individual install and build steps partially or fully redirected to the dedicated log files:

• install-apt.log
• install-pip.log
• build-docker.log

3. Various other log improvements: printing tests in the suite, printing flags passed to the frameworks, etc

Corresponding per-language buildscript PRs:

1. java - buildscripts: simplify PSM interop Kokoro buildscripts grpc-java#11121
2. cpp - [PSM Interop] Simplify PSM Interop Kokoro buildscripts grpc#36450
3. go - test/kokoro: simplify PSM Interop Kokoro buildscripts grpc-go#7171
4. node - PSM Interop: simplify Kokoro buildscripts grpc-node#2729

Notes for the reviewer:

1. The new script is backward-compatible with the old-style buildscripts.
2. Per-language buildscript PRs will be backported to all supported branches, and this should be the very last backport chore we'll ever need.
3. A minor change to the subsetting test was needed to solve a one-off exception from the normal run_test flow.

ref b/288578634

[sergiitk]

Checking LB test build:

• grpc/java/master/branch/xds_k8s_lb

[sergiitk]

URL Map @ cb47650:

• grpc/java/master/branch/xds_url_map
• grpc/java/master/branch/psm-security

[sergiitk]

LB @ f3ae087:

• grpc/java/master/branch/xds_k8s_lb