#73)

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:
```sh
# 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 - grpc/grpc-java#11121
2. cpp - grpc/grpc#36450
3. go - grpc/grpc-go#7171
4. node - grpc/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](https://github.com/grpc/grpc/blob/6e981d7460af561230d455c3623774e76e18cbfe/tools/internal_ci/linux/grpc_xds_k8s_lb.sh#L123-L127)
from the normal `run_test` flow.

ref b/288578634