[CBM] Allow specifying a list of keys that we want to restore

What is the problem?

In the tooling channel we've seen a number of questions around what is the best way to restore a limited number of documents when you know the keys. An example use case is if a limited number of documents have been corrupted and you only want to restore those.

Is there a workaround?

Yes, you can use filter-keys to only transfer the exact keys (i.e. <key 1>|<key 2>|...|<key n>. This is quite inefficient however as we still read all the documents, including their values, off disk.

What is the solution?

We should allow the user to specify a list of keys (possibly as a file) to restore. Ideally we would only read these documents off disk (i.e. by searching the index file for the keys specifically).