[Access] Add implementation for blocks, block headers and block digests data providers

[Guitarheroua]

blocks, block headers and block digest data providers

Based on the Draft design of new WebSockets, blocks, block headers and block digests data providers should be implemented as part of the new WebSocket pub/sub system. The draft design document describes all requirements and relationships between components.

Requirements:

1. Blocks, block headers and block digests data providers constructors should be implemented. Each constructor should create a corresponding subscription.Subscription based on input arguments, and store topic, subscription and other necessary parameters in a newly created instance. These constructors should be called in a DataProviderFactory::NewDataProvider method.

2. messagesHandler should be implemented for each handler type. This method will be triggered by the constructor in the separate goroutine and will run in a forever loop until the subscription is closed. This method collects streaming data from the subscription, similar to how we do it, for example in SubscribeBlockHeadersFromStartBlockID:

flow-go/access/handler.go

Lines 1181 to 1182 in 3496c0f

	sub := h.api.SubscribeBlockHeadersFromStartBlockID(stream.Context(), startBlockID, blockStatus)
	return subscription.HandleSubscription(sub, h.handleBlockHeadersResponse(stream.Send))

Then the data is formatted in response messages for the client and written to the broadcastMessage callback. The data response message structure is next:

{
    id: 'sub123',
    topic: 'blocks',
    data: [...],
}

3. The Close method should be implemented to gracefully shut down the subscription and close messagesHandler processing.

4. ID and Topic getters should return subscription.Subscription ID and topic that stores in the handler.

Tasks

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1. Implement blocks data provider and unit tests for it
2. Implement block headers data provider and unit tests for it
3. Implement block digests data provider and unit tests for it

[peterargue]

closed by #6636