add backend support for k8s platform inputs

This change adds driver support for input parameter support for the
kubernetes platform spec. Input resolution change is extracted and made
more generic so it may be re-used when building out the container spec
for the k8s config. The json parameters are unmarshalled into their
respective k8s struct.

The changes are backwards compatible for pipelines only using the
old api field spec.

Also add unit tests for constant & runtime input parameters.
TaskOutput parameter support are omitted due to a lack of appropriate
mlmd mock framework.

Signed-off-by: Humair Khan <HumairAK@users.noreply.github.com>