Add `User-Agent` HTTP header for validator client requests

[KaloyanTanev]

Currently the Lighthouse validator client does not add any identification on itself, when sending request to the upstream.

Other validator clients opted in for a header User-Agent that identified them as such: <CLIENT>/<VERSION>, i.e.: Prysm.

This would help a lot in a distributed validator middleware cases, where operators in a DV can better distinguish what their peers are running.

[nadtech-hub]

I may take on it after finishing current one

[anshalshukla]

I can take this up, please assign this to me.