[Airdrop]: Asset using VFS

[crnbarr93]

Description

The current merkle-airdrop implementation uses cw_asset to determine the airdropped asset. This is incompatible with our VFS, as such we should swap these references to use our Asset struct.

Implementation

Update the merkle airdrop contract to use our Asset struct in it's instantiation message:

pub struct InstantiateMsg {
 pub asset_info: Asset,
}

We should also take the time to add asset validation using Asset::get_verified_asset on instantiation.

Acceptance Criteria

• Ability to use VFS for CW20 references in the merkle airdrop contract
• An integration test for the above