Provide a convenient way to log arbitrary text in Drydock without needing structured log classes

Summary: Depends on D19673. Ref T13197. See PHI873.

Test Plan:
Added some code like this:

$operation->logText('Nice convenient text logging.');

...then got:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13197

Differential Revision: https://secure.phabricator.com/D19674