feat(softsign): expanded secret key can be used for signing #1349

Job	Run time
Build (stable)	3m 26s
Check	3m 11s
Build (1.74.0)	3m 39s
Validate against test harness	4m 9s
Test Suite (stable)	5m 6s
Clippy	1m 4s
Rustfmt	5s
Test Suite (1.74.0)	5m 6s
	25m 46s

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