Copybara should copy PR description into Gerrit CL

[FMorschel]

When receiving GitHub PRs for the SDK, currently copybara sends the commit to Gerrit and warns the author (see dart-lang/sdk#56746 for an example).

One thing I did notice was that even though I did add to the PR description which issue it would fix, that description is currently not copied over to Gerrit, making cases like this CL where the reviewer didn't even know there was an issue opened for the request.

I'd like to ask copybara to bring the PR description to Gerrit to help new contributors who are more familiar with GH out and make sure that if there is something like Fixes issue on the description so that whenever the CL gets merged, it would still close the issue. So there would be no more need for the author to look for the original issue and ask for someone to close it (see this example from yesterday).

[FMorschel]

Something else I noticed was that some of the time since I'm not pushing the code with the cl command, the tests will run and then I'll notice that some code was not formatted (fixed my vscode settings now) or not sorted. Maybe these tests could run when copybara pushes? Or at GH so that the contributors see this and there is less use of the Gerrit server (before Copybara pushes and maybe delaying the push to passing tests only).