Add support for ENUM column type

Vitess Connector doesn't support ENUM column type. It sends the index number instead of the ENUM value.

Vitess 9.0.0 added better ENUM support to VStream, it adds the ENUM's definition in the FIELD event:

  fields {
    name: "size"
    type: ENUM
    table: "t1"
    org_table: "t1"
    database: "vt_test_sharded_keyspace"
    org_name: "size"
    column_length: 6
    charset: 33
    flags: 256
    column_type: "enum(\'m\',\'l\',\'xl\')"
  }

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