[Access] Enable Event streaming on REST API

[peterargue]

Problem Definition

Currently, event streaming is supported only on on the gRPC APIs. This works for backend clients, and web clients using gRPC-Web.

However, the lack of REST support means FCL and the flow-js-sdk can't integrate.

Proposed Solution

Build a WebSockets handler for the REST server that implements the event streaming FLIP using the existing backend implementation.

Definition of Done

• REST API's implementation in flow-go supports event streaming
• There are unit and functional tests covering the new endpoints.