Improve robustness of `CrateAdapter::columns` method

[amotl]

activerecord-crate-adapter/lib/active_record/connection_adapters/crate_adapter.rb

Lines 132 to 138 in e875d7a

	def columns(table_name) #:nodoc:
	cols = @connection.table_structure(table_name).map do |field|
	name = dotted_name(field[2])
	CrateColumn.new(name, nil, field[4], nil)
	end
	cols
	end

This feels a bit fishy, see also CrateRuby::Client::table_structure, where select * from information_schema.columns is used, which, when used in combination, is prone to break sooner than later.

Originally posted by @amotl in #16 (comment)