Add API endpoint to get grants owned by a list of addresses

### Why Is this Needed

This is a continuation of https://github.com/gitcoinco/web/issues/6493
It's needed to find out which grants an ethereum address own. The only way to do this with #6943 is to make `N` requests (where N is number of all existing grants) to the api endpoint there and get each grant's admin and compare it to the given list of addresses. Which would be really slow and hit the server with too many requests.

### Description

Add an api endpoint that will take as input a list of ethereum addresses and as output it will give a mapping of each addresses to a list of grant identifiers.

### Definition of Done

The API endpoint to exist

### Data Requirements

I guess just its existence will be useful.