[flagd] implement contextEnricher with in-process init

[toddbaert]

add contextEnricher function param:

• converts sync_context payload object from sync response on to Evaluation context on init (requires [FEATURE] add sync_context to SyncFlags payload flagd#1635)
• stores the result from above and adds it with every evaluation, probably using a hook Java implementation
• remove e2e test filter @contextEnrichment to run existing, related e2e tests