[statsig] improve evaluation context to statsig user conversion

[beeme1mr]

Overview

The Statsig provider converts evaluation context to a Statsig user. However, it only supports directly mapping properties to what has been defined by a Statsig user. This makes interoperability impossible for providers that require a targetKey.

Proposal

### Tasks
- [ ] Map `targetingKey` to `UserID` if `UserID` is not set on the evaluation context
- [ ] Return a `NewTargetingKeyMissingResolutionError` if `UserID` is not defined on the Statsig user after the conversion.
- [ ] Map non-matching evaluation context properties under `Custom`

Resources

• https://docs.statsig.com/server/concepts/user#why-is-statsiguser-with-a-userid-required-for-server-sdks

[beeme1mr]

FYI @liran2000

[liran2000]

Yeah, it was done in Java provider, seems missing here.

[liran2000]

@beeme1mr can be resolved, via #520.