(BEEFY): Implement RPCs methods

[dimartiro]

Description

Implement the require RPCs methods for BEEFY, these are:

• beefy_getFinalizedHead - Returns the hash of the latest BEEFY finalized block
• beefy_subscribeJustifications - Returns the block finalized by BEEFY alongside its justification
• beefy_unsubscribeJustifications - Unsuscribe from beefy justifications

Dependencies

• Beefy worker keeping beefy state from which we are going to get the data to respond in this RPCs (BEEFY): Implement BEEFY voter #4177

beefy_getFinalizedHead

1. Create beefy finality API interface /dot/rpc/interfaces that should be implemented by BEEFY service /lib/beefy/beefy.go

type BeefyBlockFinalityAPI interface {
    GetBeefyFinalizedHead() common.Hash
}

2. Create a beefy rpc module /dot/rpc/modules/beefy.go

type BeefyModule struct {
    blockAPI         BlockAPI
    blockFinalityAPI BlockFinalityAPI
}

3. Create response type

type BlockHashResponse interface{}

4. Create RPC method

func (bm *BeefyModule) GetFinalizedHead(r *http.Request, req *EmptyRequest, res *BlockHashResponse) error

5. Test

beefy_subscribeJustifications & beefy_unsubscribeJustifications

1. Implement GetBeefyFinalisedNotifierChannel and FreeBeefyFinalisedNotifierChannel as part of BlockAPI

type BlockAPI interface {
    GetBeefyFinalisedNotifierChannel() chan *types.FinalisationInfo
    FreeBeefyFinalisedNotifierChannel(ch chan *types.FinalisationInfo)
}

2. Implement the required logic in /dot/state/block_notify.go
3. Create beefy listener in /dot/rpc/subscription/listeners.go
   A quick suggestion about the implementation could be

type BeefyListener struct {
    Channel       chan *types.FinalisationInfo
    wsconn        *WSConn
    subID         uint32
    done          chan struct{}
    cancel        chan struct{}
    cancelTimeout time.Duration
}

func (l *BeefyListener) Listen() {
	// Very similar than func (l *BlockFinalizedListener) Listen() but using the methods implemented in part 1.
}

4. Implement listener in /dot/rpc/subscription/websocket.go

func (c *WSConn) initBeefyBlockFinalizedListener(reqID float64, _ interface{}) (Listener, error) {
	// Similar than initBlockFinalizedListener but using the methods implemented in part 1
}

5. Register listener in /dot/rpc/suscription/suscription.go with method beefy_subscribeJustifications
6. Test