Add `SignableMsg::add_*_signature`; log extension sigs (#839) #1231

Job	Run time
Rustfmt	8s
Check	55s
Clippy	1m 4s
Build (stable)	3m 52s
Build (1.72.0)	5m 8s
Test Suite (stable)	41s
Validate against test harness	33s
Test Suite (1.72.0)	51s
	13m 12s

Provide feedback