[FR]: add function to set single claim instead of all claims `setCustomUserClaim('myClaim', value)`

**Is your feature request related to a problem? Please describe.**
When updating auth custom claims, i need to make sure i always include all existing claims, because `setCustomUserClaims` always overrides all claims. --> When multiple requests run simultaneously, we run into transactional issues (2 requests read claims, first requests updates claims with a new field, second request updates claims with another field but overrides changes from first request).

**Describe the solution you'd like**
`setCustomUserClaim('myClaim', value)` in addition to `setCustomUserClaims(allClaims)`

**Describe alternatives you've considered**
Writing claims to my own db, then regularly sync it to firebase -> this is too complicated though for something that could be solved much simpler with a new method `setCustomUserClaim`.

**Additional context**
none

Start by reading the existing setCustomUserClaims API and the custom-claims handling in the authentication SDK. Define the concurrency and replacement semantics for a setCustomUserClaim method, then add coverage for updating one claim without overwriting other claims; done means the API and its behavior are documented and tested.

Written by the indexing model from the issue text.