test(spanner): add PG.OID decoding tests

[hengfengli]

• Paramerized queries (encoding): PG.OID is passed in as the int64 in Go and will be converted to TypeCode_INT64. It does not need a specific type.
• Decoding PG.OID values: given the INT64 proto value and PG.OID proto type, we will convert it to the int64 in Go.