consider changing some SignatureTemplate methods to properties

[mr-zwets]

Currently SignatureTemplate has a method .getPublicKey() and getSignatureAlgorithm() but these could as well be a property because they do not accept any arguments.

In v0.8.0 we replaced a contract method to a property

BREAKING: Replace contract.getRedeemScriptHex() with contract.bytecode.

so this could have similar reasoning.

If getPublicKey() would accept an HD node path then it would make sense as a method I think, but the current API doesn't seem consistent

[rkalis]

Yeah we should make these properties in some future breaking release.