feat: ofep for flagd grpc sync (#45)

Signed-off-by: Kavindu Dodanduwa <[email protected]>

OFEP-flagd-grpc-sync.md

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+## OFEP: Add gRPC sync support to flagd
+
+## State: APPROVED
+
+This OFEP proposes to introduce gRPC syncs to flagd. gRPC sync will act similar to existing remote HTTP URL syncs. But
+going beyond periodic pulls, flagd can utilize gRPC server streaming to receive near real-time updates, pushed from a
+flag management system.
+
+## Background
+
+gRPC server streaming allows clients to listen and react to server-pushed data. For flagd, this perfectly matches the
+ISync interface and current sync mechanism implementations.
+
+The gRPC schema will be defined by flagd and supporting flag management system(s) will then implement the contract.
+
+<img src="images/ofep-fd-grpc-1.png" width="300">
+
+Further, grpc server push can be expanded to have `event types` such as flag additions, updates and deletions, giving more
+performant connectivity between flagd and flag management system. Performance improvements come from reduced payload
+size(single flag change vs all flags) and not having connection establishment overhead thanks to streaming.
+
+Note that the implementation complexity of `event types` lives at grpc server implementation. The implementation may use
+a state management system to derive the matching event type for a specific flag configuration change. In any case,
+flagd must not maintain any state (i.e- flagd must be stateless) and only react on the sync type to update flag 
+configurations.
+
+<img src="images/ofep-fd-grpc-2.png" width="300">
+
+### Tasks
+
+Following are the main tasks I am proposing for the implementation.
+
+- [x] POC implementation - https://github.com/open-feature/flagd/pull/297 
+- [ ] OFEP approval
+- [ ] Introduce basic grpc sync, with minimal configuration options
+- [ ] Introduce additional options, such as TLS certificates, token authentication on top of existing solution
+
+#### SSL certificates and token authentication
+
+Consider the GRPC authentication example provided through official Go guide - [Link](https://github.com/grpc/grpc-go/tree/master/examples/features/authentication)
+
+With a similar approach, it is possible to establish TLS connections and enable token based authentication/authorization
+between flagd and flag management system.