Add v0 signer process_event paths

[jferrant]

Closes #4785
I can't seem to manage to create a valid block to sign (having a hard time understanding the tenure change tx mostly), but at least I have a test for the rejection path.

Summary of the more major changes:

• Created a SignerEventTrait so that we could reuse a majority of the events.rs logic in libsigner (Enables custom SignerMessage for v0 and v1)
• Created a SignerMesssage trait and MessageSlotID trait so we could reuse the stackerdb.rs logic
• Created a Signer trait which enables generic spawning of either a v0 or v1 signer.
• Made SignerTest generic, taking any struct implementing a Signer trait to enable reuse of some testing logic.
• Changed the channel for the signers to take an enum which wraps the OperationResult and added a Status result so we can directly query the state of the signers and ensure state is as expected in tests.

[jcnelson]

Looking good so far. Mostly just some concerns about whether or not we need to bother checking the reward cycle when signing (lest the signer accidentally refuses to sign a valid block).

[hstove]

Overall this LGTM, and it's exciting to see a v0 that should be pretty much working (just needs tests). My main point of feedback is around shared StackerDB code.

So far so good :)

[hstove]

LGTM!

[jcnelson]

Overall LGTM; just minor things that are easily fixed

[hstove]

Small change, I caught this when writing new signer tests

[hstove]

LGTM!

[blockstack-devops]

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