Simplify getting the contract admin and mocking it.

[Taztingo]

Summary

Allow the user to easily query for the contract admin, and provide an easier way for a user to set the admin for mock dependencies.

Problem Definition

It is possible for a contract to use the admin within it's logic. There is currently a way to obtain the admin by querying the ContractInfo, but it's a bit clumsy. It is also very work intensive in setting up the test mock admin because the user must know how to work with the handlers.

Proposal

Create a command get_admin() that allows the user to obtain an Optional for the admin.
Create a set_admin(Option) command, update provwasm's mock_dependencies(&[Coin], Option), or create an additional mock_dependencies_with_admin(&[Coin], Option) that allows users to configure the admin while testing.

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For Admin Use

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