Ensure `SingleAttestation` conversion happens on the `BeaconProcessor`

[michaelsproul]

Description

In our SingleAttestation MVP, the conversion from SingleAttestation to Attestation happens on the main Tokio executor:

• SingleAttestation implementation #6488

We should ensure this is optimised out prior to shipping Electra. Some ideas for that here:

• https://github.com/sigp/lighthouse/pull/6488/files#r1915836564

The other option is to more wholeheartedly embrace SingleAttestation:

• SingleAttestation full implementation #6616

[michaelsproul]

I'm going to work on a small standalone PR for this.

[michaelsproul]

Fixed by:

• Optimise and refine SingleAttestation conversion #6934

Part of v7.0.0-beta.0