Ability to add works to existing list to bulk search

### Feature Request

Currently the bulk search endpoint is a handy UI to easily support adding many books to a new list. Extending this to add books to an already existing list would be a useful expansion.

### Breakdown

### Related files

Extracted from #11241 .

Refer to [this map of common Endpoints](https://github.com/internetarchive/openlibrary/wiki/Endpoints):
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### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
* [ ]

### Stakeholders

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