feat(php): support version string for gapics

[bshaffer]

We added version_string to allow for copying "common" protos which don't contain a version string. We can also use it when we want to copy files over at any point which isn't the version. This happens often, when we want a package to also contain builds of its sub-packages.

One example of this is in MapsFleetengine, where we want the build of google/maps/fleetengine/delivery to have tests, samples, and GAPIC src to be prefixed by the directory src/Discovery:

# "Discovery" subdirectory
MapsFleetengine/src/Discovery/V1/(Gapic Files)
MapsFleetengine/tests/Unit/Discovery/V1/(Test Files)
MapsFleetengine/samples/Discovery/V1/(Sample Files)
# The rest of the SDK
MapsFleetengine/src/V1/(Gapic Files)
MapsFleetengine/tests/Unit/V1/(Test Files)
MapsFleetengine/samples/V1/(Sample Files)

There are a few examples of this behavior, where we want to include generate GAPIC files, but we want to place them in a subdirectory. Other examples include:

• Bigtable/src/Admin/V2
• Firestore/src/Admin/V1
• Workflows/src/Executions/V1