Lighthouse: Read validator graffiti from file

[paulhauner]

Description

Some users have been requesting some funky graffiti requirements (e.g., drawing stuff on beaconcha.in walls, etc). I think we can satisfy practically any scenario by just reading the graffiti from a file.

Steps to resolve

• Add a --graffiti-file flag to BN, VC.
• Try to read from the file each time we produce/sign a block (not just at startup).
• If the file read fails or the value is too long, default back to --graffitti.

[michaelsproul]

I guess this would supersede https://github.com/sigp/lighthouse/pull/2044

Do you know what the intended usecase for per-validator graffiti was @pawanjay176? Perhaps we could combine both (a file with the ability to add graffiti per validator)

[GregTheGreek]

I (Lodestar) will be working on a service to update the file in a nice seamless way :)

[pawanjay176]

Do you know what the intended usecase for per-validator graffiti was

I assumed it was to sorta improve validator privacy. i.e. if 10 vals running on a single vc have the same graffiti vs each having different graffiti.

If the graffiti-file will keep getting updated for the graffiti wall, I don't think we need separate graffiti's per validator. But I might be missing some other use case here.

[GregTheGreek]

Actually that would be optimal imo. It would be nice to be able to delegate which validator pulls which graffiti..
Could the config resemble something like:

default: hi # Represents the fallback incase of an error
v1: foo
v2: bar
...