[#131] Add byte signing to pure nettest

Description

Problem: Currently there is a stub for niSignBytes in Morley.Nettest.Pure, however, this action can be used in nettest scenarios that include multisig contracts.

Solution: Optionally store secret key along with address for an alias and sign bytesequences using it.

Also add signBytes action to nettest.

Also, getPublicKey action was added to nettest. Currently, it's used only for testing signBytes, but later it can be used in morley-multisig nettest scenarios.

Related issue(s)

Resolves #131 (closed)

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