Fix GetBeaconBlock to use skip-mev-boost

[terencechain]

This PR fixes issue #12950 where GetBeaconBlock in the beacon API does not correctly utilize the skip-mev-boost parameter when making a call to the underlying prysm API.

Background:

In the beacon API, there are distinct endpoints for requesting a full block or a blind block. However, the prysm API does not make this distinction, instead relying on a skip-mev-boost parameter. The issue arises because the skip-mev-boost parameter was not being recognized or used within the GetBeaconBlock function in the beacon API. Here is the beacon API request:

v1alpha1req := &ethpbalpha.BlockRequest{
		Slot:         req.Slot,
		RandaoReveal: req.RandaoReveal,
		Graffiti:     req.Graffiti,
		SkipMevBoost: true, // Skip mev-boost and relayer network
	}

Changes:

This PR rectifies this oversight by incorporating the skip-mev-boost parameter. When this parameter is set to true, the payload construction for MEV Boost is skipped, effectively mimicking the behavior as if an override builder was applied from the engine API.