Introduce StorageLimit::ExcludedNamespace model/table

What does this MR do and why?

This MR closes: https://gitlab.com/gitlab-org/gitlab/-/issues/385723

By:

• Adding a new model for ExcludedNamespace
• Adding the new model under a new module StorageLimit
• Adding the storage_limit_excluded_namespace/namespace relation
• Covering changes with specs
• Documenting the new table addition

Migrations:

UP

main: == 20230109093043 CreateNamespacesStorageLimitExclusions: migrating ===========
main: -- create_table(:namespaces_storage_limit_exclusions, {})
main: -- quote_column_name(:reason)
main:    -> 0.0001s
main:    -> 0.0064s
main: == 20230109093043 CreateNamespacesStorageLimitExclusions: migrated (0.0076s) ==

Down

main: == 20230109093043 CreateNamespacesStorageLimitExclusions: reverting ===========
main: -- drop_table(:namespaces_storage_limit_exclusions)
main:    -> 0.0051s
main: == 20230109093043 CreateNamespacesStorageLimitExclusions: reverted (0.0064s) ==

MR acceptance checklist

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