Add extra information to the github identifiers of importer failures

What does this MR do and why?

It adds extra information to the github identifiers of importer failures (e.g. object_type) as we need to show it on the UI. It also edits the Gitlab::GithubImport::BulkImporting module to store the github_identifiers inside the Labels, Milestones and Releases importers failures.

It is the first part for the GitHub importer: respond with detailed errors info. The second part - !117133 (merged).

Screenshots or screen recordings

If the importer failed to import a pull request	If the importer failed to import a label

If the importer failed to import a protected branch	If the importer failed to import a collaborator

How to set up and validate locally

1. Import a GitHub repo via UI (on the /import/github/status page) or GitLab API. Import has to finish with the Partial status (e.g. if the repo has an issue, raise an exception inside Gitlab::GithubImport::Importer::IssueImporter)
2. Ensure the last error stored in the import_failures table that is related to one not imported entity contains the new entity details (such as object_type) in the external_identifiers column (check project.import_failures relation in the console).

MR acceptance checklist

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