Add comment-attribute to database.xml-scheme

MySQL supports comments for tables and columns:
Search for "Comment" on https://dev.mysql.com/doc/refman/8.0/en/create-table.html

Comments are very helpful for developers to understand what a certain table or column is about.

By extending the database.xml-scheme with a comment-attribute for <field>- and <table>-tags it should be easy to allow developers to add their comments to tables and columns.

What do you think, @henbug & @mor ?