Support User-Defined Function

`enable_user_defined_functions` is currently disabled in the feature flags, but is there a specific reason for this?

https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/blob/d42ed938ace7628e6324f0a8eb967b8fdfb3b3f8/common/feature_flags.h#L48

Although there is no mention of UDFs in the Spanner reference documentation yet, the following UDF package is already considered Generally Available (GA), and I have confirmed that it works on production Spanner instances.

[googleapis/spanner-sql-udf](https://github.com/googleapis/spanner-sql-udf/blob/3cf35f53647d8e21b17bcf1de31618d016770770/README.md#status-of-udf-package-availability)

> This UDF package is Generally Available (GA) for use on Cloud Spanner.