lp-propose: allow user to specify the number of reviewers to assign

[Gauthier Jolly]

Depending on the workflow, it's common for a project to require more than one review/approval per MP. It would be nice to have a flag to specify the number of reviewers to assign to the MP on creation.

Launchpad and launchpadlib allow this by doing:

branch_to_merge.createMergeProposal(merge_target=main_branch, commit_message='...', reviewers=[repo.owner_link]*nb_reviewers, review_types=['']*nb_reviewers, needs_review=True)

If the owner is a person, then the MP is created with only one reviewer no matter how many time it is specified. If the owner is a team, it will work as expected.

The flag could be `--reviewers` or `--approvals/-a`.

I'd be happy to open an MP if this feature is accepted.