Support multi-column indexes on OpenFileGDB

[apjoseph]

Feature description

Currently creating multicolumn indexes with the OpenFIleGDB driver on SQLite or ORGSQL dialects is not supported -despite being supported by the format itself.

CREATE INDEX field1_field2_idx ON table (field1,field2) 

will fail with "field1,field2 does not exist". Given the error message, I am not sure if the issue lies with the sql parser or is a limitation of the OpenFileGDB driver itself -or both.

Additional context

No response

[rouault]

This is both, but the main limitation comes from the index creation capabilities of the OpenFileGDB driver itself. Unlikely to be implemented without funding (besides coding, it might require additional reverse engineering to figure out how this is implemented)
In the meantime I've queued #10591 to emit an explicit error.