Add Nimbus Version to BuilderAPI requests

[colink14110476]

Is your feature request related to a problem? Please describe.
For debugging and missed slot investigation within the builderAPI pipeline it would be great to include the nimbus version within the user-agent. The reason this is helpful is because this can help entities downstream like (mev-boost and relays) identify issues earlier.

The particular requests this would be helpful on include:

	pathRegisterValidator = "/eth/v1/builder/validators"
	pathGetHeaderPrefix   = "/eth/v1/builder/header"
	pathGetPayload        = "/eth/v1/builder/blinded_blocks"

Other beacon clients currently apply a user agent string like so:
User-Agent=Prysm/v6.0.2/2ec1ef53dcb114da22698e8ccd9bc1e3aa8e3870

Nimbus's presents like so:
User-Agent=nim-presto/0.0.3

Describe the solution you'd like
It'd be great to have the user agent show up like so:
User-Agent=nim-presto/v25.5.0

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context