Allow voluntary exit command to write to file rather than publish

[nflaig]

Teku implemented this recently https://github.com/Consensys/teku/pull/8168 and it should be quite easy to add in Lodestar as well.

Seems useful to have since with EIP-7044 signed voluntary exit messages are perpetually valid and more people might wanna prepare exit messages without actually publishing them.

The alternative to this right now is to use signVoluntaryExit API of the keymanager or external tooling.