Write share to `--consensus.signing-share`

[SuperFluffy]

Describe the feature

The share passed in through --consensus.signig-share becomes pointless at the end of an epoch. The node should override it with the latest signing share obtained from the on-chain DKG ceremony.

Following from this, we should also not store the share in the in-memory DKG actor state.

Additional context

No response

[SuperFluffy]

Will not implement this because would immediately run into atomicity issues: upon resolving a DKG loop, we are currently persisting the public outcome and private share in one atomic operation. If we instead split this (write the public outcome and share to separate files), one of the operations could fail, which means that on restart we'd need to reconcile the mismatch.

While that's in theory possible, it's easy to get wrong and unnecessary.

If desired, we can provide a utility to extract the encrypted share.