feat: add support for PG.OID in parameterized queries

[larkee]

This PR adds support for using PG.OID values in parameterized SELECT queries.

For example:

with database.snapshot() as snapshot:
  results = snapshot.execute_sql(
    "SELECT $1",
    params = {"p1": 123},
    param_types = {"p1": spanner.param_types.PG_OID})
    for row in results:
      print(row)