Add optional executor restriction to cw3-flex

[ethanfrey]

In cw3-flex-multisig, anyone can execute the proposal once the vote has passed. This is not always ideal.

I would add a backwards-compatible executor: Option<Executor> to both InstantiateMsg and Config. This could accept the following values:

• None - same behavior as now (and same struct)
• Some(Member{}) - any member in the voting group (even with 0 points) can execute the contract, same restriction as those who can propose
• Some(Only{address}) - only the given address can execute the contract

Use case for Member is eg. a swap from a multisig, where you want control of the timing (not someone using a sandwich attack on your trade).

Use case for Only is a separate approval / veto step. This is like the US President needing to sign a law once it passed congress. Or even the two houses. It can allow more complex governance structures.