Re-using a session pool on connect()

[waiho-gumloop]

Is your feature request related to a problem? Please describe.
When using SQL Alchemy with python-spanner-sqlalchemy, I've noticed that we're running .bind(), instantiating the database object and executing BatchCreateSessions for every single connect() call, which adds quite a bit of additional overhead before the query even executes. Main overhead that I saw was BatchCreateSessions when using a PingingPool.

Describe the solution you'd like
Would like the ability to pass in an existing database object (or have it be idempotent/detect that there's already a pool) so that the same pool and its pre-created sessions can be re-used for every SQL alchemy connection checkout, similar to how using the raw spanner client works.

Describe alternatives you've considered
I've tried passing a pool to the connect(), but it still recreates the sessions.

Draft PR: https://github.com/googleapis/python-spanner/pull/1493/changes

While this PR works for my specific use case (SQL Alchemy x python-spanner-sqlalchemy + PingingPool), I am not clear of the implications of allowing the re-use of the database object. Would appreciate any guidance here

PR contains a benchmarking script that tests a 7 combinations (raw spanner client, mix of values for QueuePool/StaticPool x Spanner's PingingPool):

[waiho-gumloop]

This seems to work as well (for my specific SQL Alchemy use case), without breaking the interface of this library.

Fails for anything using the database object, since new databases are getting created

class SharedPingingPool(spanner.PingingPool):
    """PingingPool subclass with idempotent bind() for sharing across connect() calls.
    
    First bind() triggers BatchCreateSessions; subsequent calls are no-ops.
    Upstream connect() sets _own_pool=False when pool= is provided, so
    close() won't destroy the shared pool.
    """
    
    def bind(self, database):
        if self._database is not None:
            return  # Already bound — skip BatchCreateSessions
        super().bind(database)