Access control for signatures based on params in operation

[jevonearth]

Ability to add smart contract methods (extracted from params) to an approve_list or deny_list

This feature will allow smart contract owners granular control on the types and properties of operations they sign using signatory.

To implement this, signatory will need to have the ability to parse and inspect Michelson. The operator will want to write some form of rules that apply to the entry points.

We can consider this issue blocked until we have the ability to parse Michelson into an AST in go.

Blocked by #191

[AbineshECAD]

@jevonearth Need inputs on the status of this issue.