Create draft merge requests for remaining tables without sharding keys

So far we have managed to automatically determine the schema and sharding key for ~80% of tables. The remaining ~20% will require manual input.

For the remaining tables, create Draft merge requests that partially update the database dictionary entries, and prompt the assignee/reviewer to choose one of the following for each entry:

• Add a sharding key
• Add a desired sharding key
• Set the schema to gitlab_main_clusterwide
• Create a follow-up issue (or link an existing one) for the entry if none of the above options are currently possible