Add support for minimal preset

[barnabasbusa]

Right now if a chain uses minimal preset we get:

time="2025-11-28T12:21:19Z" level=error msg="Failed to check for new serving checkpoint" error="failed to fetch bundle: failed to decode electra signed block contents\ninvalid ssz encoding. first variable element offset indexes into fixed value data" module=beacon/default

Would be good to automatically/with a flag we could set preset to be minimal.

[barnabasbusa]

Maybe we can use @pk910's dynamic ssz for this.