[REFACTOR] Add type feild to sync.SyncFlags payload.

[toddbaert]

of course i can elaborate. Currently within the RPC mode we do have the EventStream, which will send on the inital connection a message provider_ready, and for upcoming messages configuration_change. This allows us within the RPC mode to be stateless. As flagd anyways knows when a configuration is established, or when it only informs about updates.

see https://buf.build/open-feature/flagd/docs/main:flagd.evaluation.v1#flagd.evaluation.v1.EventStreamResponse

this would allow us to unify the logic, and keep both modes stateless. Currently we need to know, if we have been connected or not as we need to know, if we need to fire a configuration change or provider ready event.

I hope i could shed some light here, and i hope my motivation regarding this unification is clear

Originally posted by @aepfli in #1584

Requirements

• update proto; add new field to Sync Response type, which carries same data as the type in https://buf.build/open-feature/flagd/docs/main:flagd.evaluation.v1#flagd.evaluation.v1.EventStreamResponse - on first stream payload, it should be provider_ready on subsequent payloads, it should be configuration_change feat!: add sync stream type and context flagd-schemas#183
• refactor providers to use the new metadata field, but also use the old RPC for now (new field should "win")
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