Allow overriding Bitbucket clone URL user for API Token support

[Likaj]

We are using Scala Steward with Bitbucket and currently authenticate using an App Password. However, Bitbucket will deprecate App Passwords in 2026 in favor of API Tokens.

Scala Steward uses the following format for cloning repositories:
https://{login}:{password}@bitbucket.org/{workspace}/{repo}.git

When we substitute {login} with an email address and {password} with an API token, cloning fails — because Bitbucket's Git interface does not support email addresses as usernames when using API tokens.

It requires the actual Bitbucket username like this:
https://{bitbucket_username}:{api_token}@bitbucket.org/{workspace}/{repo}.git
Because of this, cloning fails with API Tokens.

Request: add support for a config parameter (e.g. cloneUser or bitbucketUsername) that overrides the username part in the clone URL.

This would help users transition to API Tokens before the deprecation deadline.

Thanks in advance!