Adding author field into system.mutations

[EilRoviSoft]

Changelog category:

• New Feature

Changelog entry:

Added author column into system.mutations table.
...

Documentation entry for user-facing changes

• Motivation:
  Sometimes it is needed to track what user is initiated mutation query.

• Example use:

SELECT database, table, author FROM system.mutations;

[alexey-milovidov]

Please edit the title and use the description template.

[alexey-milovidov]

Needs a test.