Set user agent for builder calls

[michaelsproul]

Description

The MEV relays analyse calls based on their HTTP user agent, which Lighthouse currently does not set. In the incident immediately after Capella, relays used the user-agent to block faulty Prysm nodes from requesting blocks and triggering failures.

Expected Behaviour

• Add a default user agent header like Lighthouse/vX.Y.Z to each builder request.
• (Optional) Add a CLI flag to configure this user agent

Note that when connecting to relays via mev-boost it will add its own version as a prefix of the CL supplied user agent, so we will often end up with a user agent like mev-boost/v1.5.0 Lighthouse/v4.1.0.

[michaelsproul]

I'll take this

[michaelsproul]

Closed by #4199