[FEATURE] Support for advanced sync provider configuration

[toddbaert]

Requirements

flagd supports various sync providers (filepath, kubernetes, http, grpc), and also supports the usage of multiple providers simultaneously (just as an example, flagd can be started to listen to an HTTP endpoint, watch a file, and connect to 2 gRPC servers).

Each of these providers may require individual configuration. For instance, each gRPC stream might need different certs, or http syncs may each require different auth headers. Once flagd supports this sort of configuration, we need to be able to define it in CRDs.

Blocked by: #405