spanner: handle unrecognized database default isolation level in PG SHOW statements #14967

Follow-up to #14904, based on this review comment.

If the database's default_transaction_isolation holds a value this client doesn't recognize, the PostgreSQL isolation level SHOW statements fail with FAILED_PRECONDITION. An older client should keep working with a database that uses a newer default isolation level.

Proposal:

• Store the raw default_transaction_isolation value in DatabaseMetadata and expose it through DatabaseClient.
• Return that value from the SHOW statements when the level is UNRECOGNIZED, instead of throwing.