React: support multiple contexts

The core sdk allows you to use multiple flagsmith instances via creatFlagsmithInstance(), however the provider only allows for 1 single instance. Instead we should allow defining a "domain" as openfeature calls it.

Proposed syntax: note this suggestion adds an overload where useFlags takes an object rather than passing yet another parameter, we can keep this backwards compatible.

```
// since openfeature calls it a domain

useFlags({flags:[],domain:"a"})
useFlagsmith({domain:"a"})
```