Set User-Agent header for any outbound http requests

[pinebit]

💎 Issue

Prysm VC needs to change User-Agent header to properly report the VC's identity: name and version, instead of Go-http-client/1.1.

Background

We would like to unify all VC clients behavior here. Currently at least Lodestar and Lighthose properly report the client names and versions.
This is the ask for Teku VC to implement the same: Consensys/teku#9602

Description

Having proper User-Agent would allow upstream software (such as DV middleware) to properly identify the client's identity. This is priceless for observability where system design allows for various VC clients.

[trinadh61]

Hi @rkapka can I work on this, also is there anything else I can refer to as example

[OisinKyne]

@trinadh61 If you're looking for a format, something like lodestar's from this screenshot might work, or what prysm previously put in unspecified graffiti flags might be good suggestions :)