Support gitlab_mr_destination

[dpoluyanov]

I've working on support gitlab-related workflows for our company and implemented initial support for gitlab_mr_destination,
It is pretty initial work and mostly based on #214 in ground, but still could be reviewed, while I'm working on tests and documentation update for it

Another step toward #153

Closes #214

[dpoluyanov]

@mikelalcon, could you please take a look and leave some remarks on it?

[mikelalcon]

Hi! Thanks for the contribution!

I see that some of the classes (like java/com/google/copybara/git/GitLabMrWriteHook.java) are copies of the GH one. Is it possible to extend Gitlab one from the GitHub one instead?

Regarding to #214, we are currently refactoring this code to allow arbitrary flags but that they can be validated. Give us 1-2 weeks :)

[dpoluyanov]

Yes, most of classes the same as in github package and probably some code could be extensively reused for github and gitlab integrations but for start I’ve intentionally kept it separate in order to keep it clean and not break internal code at google.
If it’s not a point, I can extend or create common classes for both integrations wherever it suitable

[mikelalcon]

I think we could have some shared code. I can try to do a refactor (including your code) if it SGTY

[dpoluyanov]

Sounds good, but let me finish pull request =)

[abitrolly]

So does copybara support GitLab?