[flagd-provider] add events support

[toddbaert]

Once the dotnet-sdk has events and init/shutdown support, we need to update the flagd-provider to emit events and do proper init/shutdown. We already have a stream implemented for cache-busing, so this should be pretty simple.

Definition of done:

• flagd does does startup logic (like initializing it's stream) in initialize function
• flagd shuts down stream in close/shutdown function
• flagd emits appropriate events on flag changes, and errors
• associated testing

Blocked by:

• dotnet-sdk implementation: Changes to support spec v0.6.0, v0.7.0 compliance dotnet-sdk#126