Add comment and table_comment table schema definition

[mechnicov]

Motivation / Background

This Pull Request has been created because these methods are missing, but could be helpful for bulk changes of the table. This will allow you to add comment(s) to the table and/or to the column(s) using single block, improving readability and performance because of single query

Detail

This Pull Request adds comment and table_comment for change_table definition

change_table :users, bulk: true do |t|
  t.comment :login, "Username"
  t.table_comment "Usual users"
end

Additional information

This was intended as non-breaking change

Checklist

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[mechnicov]

May be t.column_comment (or just t.comment) and t.table_comment is better

[mechnicov]

Hello @rafaelfranca

I see you merged similar commit

f84e1eb

Please review my PR as well. Thank you

[zzak]

I don't think this is the same as validate_check_constraint, and is not supported for all adapters.

[mechnicov]

Hello @zzak. Thank you for your reply. Currently in the abstract adapter there are exceptions when comments are not supported and specific methods for PostgreSQL and MySQL 1, 2 adapters

Maybe you have some idea how to improve PR changes?

[mechnicov]

I've added condition for tests (only for adapters that support comments). As for the lib code, the issue with adapters that don't support comments will be resolved at the schema statement level (as I wrote above, there will be exceptions if the appropriate methods are not defined in the specific adapter). I've also changed naming (t.comment rather than t.change_column_comment looks like the rest of existing methods)

@zzak could you look at the PR?