[FR] - bulk update/edit auth users

**Is your feature request related to a problem? Please describe.**
It is not currently possible to bulk update a large amount of users customClaims in a single request, using a js map makes lots of requests and reaches rate limits very quickly. e.g
`users.map(user => firebaseAdmin.auth().setCustomUserClaims(user.uid, user.claims));`

Removing users then re-importing with bulkImport would logout all current users from web/mobile apps, it would also require users to set new passwords again so this isn't an option.

**Describe the solution you'd like**
add bulkUpdate or bulkEdit command that matches users by uid or email and updates matching records

**Describe alternatives you've considered**
putting updates onto a queue and processing them individually within the firebase customClaims rate limits

**Additional context**
even the ability to run small bulk updates such as 10 or 100 would be better than processing individually

example use case:
B2B with several "clients" all in one firebase account. A client gets an upgrade and all the users gain a new feature, a new attribute needs adding to each users custom claims

Start by reviewing the existing firebaseAdmin.auth().setCustomUserClaims and bulkImport usage described in the issue. Determine the supported matching and update semantics for a bulk operation, including practical batch sizes and rate-limit behavior. Done means a documented bulk update/edit capability can update matching users without deleting accounts or resetting passwords, with tests covering the requested behavior.

Written by the indexing model from the issue text.