Draft: Exclude projects or groups from CI Job Token authorizations compaction

What does this MR do and why?

A new requirement has been added as part of [BE] Provide mechanism to migrate authenticatio... (#478540) that any existing, user generated, entries in the allowlist should be factored in to the compaction process. To accomplish this, this MR proposes a new parameter to the Ci::JobToken::AuthorizationsCompactor called excluded_namespace_paths, which will contain an array of traversal_ids of the existing projects or groups in the allowlist.

Then, in the process of generating the origin_project_traversal_ids, any project that is part of an excluded path will be removed from the list (because that authorization is already provided as part of an existing entry). This check is performed in namespace_path_excluded?.

The filtered origin_project_traversal_ids is then passed to Gitlab::Utils::TraversalIdCompactor. The limit should also be decreased by the total number of existing entries in order to ensure the compacted allowlist remains within the required limit.

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