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

[patricsteiner]

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

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