Improve GitLab migration logs

In the past, some improvements were done in the GitLab Migration logs, but we still can make it better since it's the best tool we have to debug problems.

Proposed improvements

• Add a text message key to all logs - It's important to have a message because it explains what the log is about and can be used as a filter (!100734 (merged))
• Use the same attribute name to all logs - Attribute names aren't consistent. For example sometimes bulk_import_entity_id is used and sometimes entity_id (!100307 (merged) )
• All the logs should include the bulk_import_id key - This attribute can be used to filter all logs for a specific import (!100611 (merged))
• [ ] Log the size of the group/project and source version at the beginning of the process (Not sure if it's possible)
• Log the source version (e.g 14.2) and GitLab version (ee foss) at the beginning of the process (!100917 (merged) )
• Add stack trace to the error logs - This will allow us to detect exactly where the error happened (!100917 (merged) )
• Include the key "importer: gitlab_migration" to all logs - This attribute will allow us to filter only by GitLab migration logs (!100734 (merged))
• Log the imported project URL as proposed in #375776 (closed) (!100917 (merged) )
• Create follow-up issue once all tasks here are complete to implement creating a logger similar to the GitHub::Logger. We could set as many universal bulk_import log attributes as possible in this new logger (eg importer), then change BulkImport to use it. (#378468 (closed))