Add signerverifiers for RSAPSS, ECDSA, ED25519

[adityasaky]

This creates DSSE friendly signerverifiers for the these key types, with some of the logic inspired by in-toto-golang's keylib. The idea is to move this in here and use it for Metablock and DSSE in in-toto-golang going forward.

[coveralls]

Pull Request Test Coverage Report for Build 4853551288

• 204 of 260 (78.46%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-6.9%) to 84.77%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
signerverifier/ed25519.go	37	45	82.22%
signerverifier/ecdsa.go	42	57	73.68%
signerverifier/utils.go	63	78	80.77%
signerverifier/rsa.go	62	80	77.5%
<!--	Total:	204	260	78.46%	-->

Totals
Change from base Build 4658330519:	-6.9%
Covered Lines:	423
Relevant Lines:	499

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💛 - Coveralls

[adityasaky]

@pxp928