Specify dialect per external model

[markgraphene]

Issue
As per the duckdb example configuration (https://sqlmesh.readthedocs.io/en/stable/integrations/engines/duckdb/?h=duck#duckdb-catalogs-example) it is possible to attach a variety of data catalogs, however if specifying these as external tables there is no way to specify the dialect for each external table - ie. you may have an external model sqlite.main.example and another model postgres.public.example2, which should each be specified as per their dialect.
Currently the only way to specify the dialect for external tables is via model_defaults.

gateways:
  my_gateway:
    connection:
      type: duckdb
      catalogs:
        memory: ':memory:'
        sqlite:
          type: sqlite
          path: 'test.db'
        postgres:
          type: postgres
          path: 'dbname=postgres user=postgres host=127.0.0.1'
          read_only: true

[markgraphene]

completed here - #3136