Add support for SET column type

Vitess Connector doesn't support the SET column type. It sends the index number instead of the comma-separated SET values.

Vitess 9.0.0 added better SET support to the VStream API, it adds the permitted values in SET's definition in the FIELD event:

  fields {
    name: "set_col"
    type: SET
    table: "t1"
    org_table: "t1"
    database: "vt_test_sharded_keyspace"
    org_name: "set_col"
    column_length: 21
    charset: 33
    flags: 2048
    column_type: "set(\'a\',\'b\',\'c\',\'d\')"
  }

The Vitess Connector can now parse the column_type for the list of permitted values and store it in the in-memory schema of the table.