Prysmctl: generate genesis state with correct extra data

[terencechain]

h/t to @yahgwai for the report

Extradata specified in the execution layer's genesis JSON can be arbitrary in length. However, when used to generate an execution header for the beacon state, we made a deliberate choice to truncate it to 32 bytes instead of leaving it empty. This behavior has been in place since #11809.

#14351 removed the extra data truncation, causing genesis state generation to fail as follows:

INFO[0000] Specified a chain config file: /Users/t/go/src/github.com/prysmaticlabs/prysm/config.yml  prefix=genesis
INFO[0000] No genesis time specified, defaulting to now()  prefix=genesis
INFO[0000] Delaying genesis 1736981389 by 15 seconds     prefix=genesis
INFO[0000] Genesis is now 1736981404                     prefix=genesis
INFO[0000] Setting fork geth times                       cancun=1736981404 prefix=genesis shanghai=1736981404
118
FATA[0000] Could not generate beacon chain genesis state  error="--.ExtraData (bytes array does not have the correct length): expected 32 and 118 found" prefix=genesis

This PR reverts the changes related to extra data truncation.