[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

首先阅读现有的 setCustomUserClaims API 以及 Authentication SDK 中对 custom claims 的处理。定义 setCustomUserClaim 方法的并发和替换语义，然后添加更新单个 claim 而不覆盖其他 claims 的测试覆盖；当 API 及其行为都有文档说明并经过测试时，即视为完成。

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