Parse STRUCT in CREATE MODEL

[apstndb]

STRUCT can be used in CREATE MODEL, but it can't be parsed because CreateModelColumn uses SchemaType and StructType doesn't implement SchemaType.

CREATE MODEL MODEL_NAME
INPUT(content STRING(MAX))
OUTPUT(
  embeddings
    STRUCT<
      statistics STRUCT<truncated BOOL, token_count FLOAT64>,
      values ARRAY<FLOAT64>>
)
REMOTE OPTIONS (
  endpoint = '//aiplatform.googleapis.com/projects/PROJECT/locations/LOCATION/publishers/google/models/textembedding-gecko$MODEL_VERSION'
)

I think StructType should also be a SchemaType. Validation is a responsibility of analyzer.

Note: column_type() of cloud-spanner-emulator includes STRUCT.

https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/blob/7ad3d598613bc6fef5ef9806bb5a3ee343f15586/backend/schema/parser/ddl_parser.jjt#L337-L356