Allow customization of platforms, options, etc. for the precheck dry-run

The precheck button could allow for some degree of customization regarding how the CI dry-run should happen.

For instance, one may want to select:

• only a subset of platforms to build upon
• whether to checkout or cherry-pick the commit on top of the target branch
• whether to build only, or build+run tests

And so on.

Probably adding a full blown UI into Gerrit is overkill for this task, but how about having a Precheck Bot that one can add to the review, and then leave a comment to trigger the precheck run?

E.g.

bot: precheck
platforms: win32, linux
onlybuild
cherry-pick

(We'll find the right syntax after we choose the perfect color for the bike shed)

Kudos to alexandru.croitor for the idea.