[cw-proposal-single] Allow limiting who can execute contracts

[ethanfrey]

The goal is to allow the WYND DAO to vote as normal to pass proposals. But it also needs the approval of the wynd foundation before they are executed.

The simplest way is to only allow the WYND foundation to execute proposals, which must have been previously passed by the WYND DAO. DAO DAO v2 adds some more complex authorisation flows, but this is good enough for the v1 release.

@ueco-jb implemented something similar in CosmWasm/cw-plus#741 for cw3-flex. We just need to port the basic idea over into this repo and update tests.

This only updated cw-proposal-single