IF: Unification: Sign finalizer_digest of received block header and send out vote

[arhag]

First validate the block header and use it to compute the next block_header_state. The block_header_state should compute the finalizer_digest. This should be done as part of receiving the block.

Afterward, without blocking the signal of receiving the block, the configured finalizers of the node should attempt to sign that block asynchronously, either the finalizer_digest or the weak digest computed from the finalizer_digest and a tag indicating WEAK, depending on whether the finalizer chooses to sign strong or weak, respectively. A new signal will need to be added to let the net_plugin know there is a new vote message that is ready to broadcast.

For this issue, the finalizer should always sign strongly. This will be refined in a later issue.