[stacks-signer] Write pending transactions to stacker db slot for miners to observe

[jferrant]

Description

Update signers to store a list of txids in stacker db for signer generated 0 fee STX transactions

Context

Miners need to know how to construct a block that signers will sign. They will need to observe the signer pox-4 stacker db instance and try to construct a valid block that includes the tx's that signers are compelling them to.

Expected Approach

Update stacker db slots to include on for a list of txids Ensure that every transaction broadcasted to the mempool is written to stacker db

Acceptance Criteria

• [ ] PR adds the feature
• [ ] PR tests that these items are serialized and deserialized from stacker-db correctly