Spanner: Add PingingPool to toplevel spanner module

[crwilcox]

No description provided.

[crwilcox]

The documentation at https://gcloud-python.readthedocs.io/en/stable/spanner/session-api.html#google.cloud.spanner.pool.PingingPool (not this clients but reasonable enough) states you can access it this way but you cannot as it was not included.

[tseaver]

We should probably be adding the other pool classes, too (FixedSizePool is there already):

• AbstractSessionPool (users who write their own pool implementations need it.
• BurstyPool
• TransactionPingingPool

[crwilcox]

Current list of impls is below. I can add TransactionPingingPool as well

__all__ = (
    "__version__",
    "AbstractSessionPool",
    "BurstyPool",
    "Client",
    "COMMIT_TIMESTAMP",
    "enums",
    "FixedSizePool",
    "KeyRange",
    "KeySet",
    "param_types",
    "PingingPool",
    "types",
)