integrity: signer and verifier implementation

[patapenka-alexey]

Closes #TNTP-4171

[coveralls]

Pull Request Test Coverage Report for Build 19289323901

Details

• 30 of 53 (56.6%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.0%) to 23.077%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crypto/rsa.go	26	37	70.27%
marshaller/marshaller.go	0	12	0.0%

Totals
Change from base Build 19101577872:	1.0%
Covered Lines:	414
Relevant Lines:	1794

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💛 - Coveralls

[bigbes]

Last thing: add tests for running without privkey, pubkey and both pubkey/privkey.

1. without privkey is needed to check that verification only works and signer will return error
2. without pubkey - signer will work and verification wont.
3. without both of them - none of the above operations will work.