feat(softsign): expanded secret key can be used for signing #1322

Job	Run time
Build (stable)	1m 39s
Clippy	1m 0s
Build (1.74.0)	2m 32s
Rustfmt	15s
Test Suite (stable)	1m 37s
Test Suite (1.74.0)	1m 29s
Check	1m 10s
Validate against test harness	1m 34s
	11m 16s

Provide feedback