Add assertions for statically configured attributes

[toddbaert]

With open-feature/flagd#1448, we can now inject arbitrary key/values into the evaluation. These are injected into flagd's evaluation in rpc mode, and delivered over the GetSyncMetadata call in in-process mode (and subsequently added to the provider's local evaluation).

We should add a -X arg here that adds some property, and then write a flag that makes use of that property in a rule, and lastly add an assertion about this flag in a test suite (probably flagd-json-evaluator.feature). This will ensure that the flag evaluates consistently when evaluated in all modes.