Add beacon block reward endpoint

[MisiakGeo]

Add Block Rewards.
Endpoint /eth/v1/beacon/rewards/blocks is now available to fetch data regarding the block rewards on a slot.

To test it you can run the following:

package main

import (
	"context"
	"errors"
	"fmt"
	"os"
	"time"

	"github.com/attestantio/go-eth2-client/api"
	"github.com/attestantio/go-eth2-client/http"
	"github.com/davecgh/go-spew/spew"
	"github.com/rs/zerolog"
)

func main() {
	// Provide a cancellable context to the creation function.
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	var err error
	client, err := http.New(ctx,
		// WithAddress supplies the address of the beacon node, as a URL.
		http.WithAddress(os.Getenv("BEACON_NODE_ADDRESS")),
		// LogLevel supplies the level of logging to carry out.
		http.WithLogLevel(zerolog.Disabled),
	)
	if err != nil {
		panic(err)
	}

	fmt.Printf("Connected to %s\n", client.Name())

	httpClient := client.(*http.Service)
	reward, err := httpClient.BeaconBlockReward(ctx, &api.BeaconBlockRewardOpts{
		Common: api.CommonOpts{
			Timeout: 2 * time.Minute,
		},
		Block: "head",
	})
	if err != nil {
		var apiErr *api.Error
		if errors.As(err, &apiErr) {
			switch apiErr.StatusCode {
			case 400:
				// Bad Request
			case 404:
				// No block found.
			case 503:
				// Node is syncing.
			case 500:
				// Internal Server Error.
			}
		}
		spew.Dump(err)
		panic(err)
	}
	spew.Dump(reward.Data)

	// Cancelling the context passed to New() frees up resources held by the
	// client, closes connections, clears handlers, etc.
	cancel()
}

One problem I had, was when testing it against QuickNode, some time an error appeared Context Deadline Exceeded, due to the fact that the rewards endpoint with curl takes about 1 min to complete.

[MisiakGeo]

Ignore the events.go (It was fixed by @mcdee). It is still working perfectly without errors, but urlForCall function is considered better.

[mcdee]

Thanks for this PR, a few comments inline. Also, please could you add in api/v1/beaconblockreward_yaml.go and api/v1/beaconblockreward_test.go files to add YAML support and tests for the spec, respectively? The tests in particular are important to keep up coverage rates for the library.

[mcdee]

Please could you fix this comment to match the function.

[mcdee]

Please can you fix comments so that they end in a period? Thanks.

[mcdee]

This should be of type phase0.ValidatorIndex

[mcdee]

This and other gwei values should be of type phase0.Gwei

[mcdee]

Please can you revert the changes to this file, as they have been addressed elsewhere.

[MisiakGeo]

@mcdee Thank you very for the comments. I will take a look. A comment from my side. YAML files can be found in api/v1/daneb/*_yaml.go . Should I place them in api/v1?

[mcdee]

Sorry, after taking another look it appears that the rewards object is not a spec object, so it doesn't need the YAML piece.