feat: presigned validator exit txn

[hamdiallam]

Description

Would like to store a presigned validator exit transaction as a last resort mechanism for publishing an exit.

Rational

• Revealed presigned txn will result in potential unwanted exit. This okay as the validator can generate new keys and redeposit since the withdrawal address should always be set
• Can be a bit more liberal with how this presigned txns is backed up and accessed as a last resort mechanism

Present Behaviour

$ lighthouse account validator exit only supports sign + broadcast the withdrawal txn

Added Behaviour

Added flag to log and exit the withdrawal txn

$ lighthouse account validator exit --presign
0x...
``

[michaelsproul]

We are soft-deprecating lighthouse account commands in favour of lighthouse validator-manager ones. The PR for exits in the validator manager will be merged soon:

• Add voluntary exit via validator manager #6612

I think we should add this feature to that branch

[hamdiallam]

@michaelsproul thanks for the quick reply!

sounds great, i'll take a look at that branch