Stacks signer: Track bitcoin block timing to allow for a miner to extend

[kantai]

If a bitcoin block takes a sufficiently long time to mine, nakamoto miners should be allowed to create a TenureExtend transaction. Signers can then check if this allowed via their own tracking.

[kantai]

Note: this isn't strictly a 3.0-must. This is a non-consensus-changing feature that would improve performance only in situations where bitcoin blocks last longer than 10 minutes.