restrict_merges branch restriction requires "Bitbucket Username"

[twoodhouse]

I spent a while wrestling with assigning users to the "restrict_merges" branch restriction before realizing that the go library used by this provider can only create user objects within a branch restriction based on those users' Bitbucket usernames. Atlassian account id or UUID will not work.

https://github.com/ktrysmt/go-bitbucket/blob/master/branchrestrictions.go#L127

For automation, this is less usable than the other IDs since Bitbucket Cloud doesn't allow anyone (even admins) to retrieve the Bitbucket Username of a user other than themselves.