Prysm agent details in default graffiti

[kasey]

🚀 Feature Request

Description

By default many other clients include the client name + version/build info in the graffiti area of a proposed block, while still allowing the user to override this value with a flag.

Knowing the version of the client software that produced a block can be helpful for debugging. This also sets us up to align with a proposed engine api change enabling clients to easily include CL+EL identifiers in blocks, which will aid in analyzing the health of EL client diversity.

Describe the solution you'd like

Set the default graffiti to include the "prysm" agent identifier along with version/build information. We already have the option to customize graffiti, so we only need to change the default to include prysm build info.