Implement asset recovery mechanism

[smol-ninja]

^{Originally discussed in https://github.com/sablier-labs/company-discussions/discussions/11}

Implement a mechanism to recover any surplus amount of assets from the core contract. The following approach has been discussed at length in the original discussion:

1. Define a single variable to track down the amount of assets that the protocol owes to the users: mapping (erc20 => amount). This variable should be updated whenever there is a transfer of asset into/out of the protocol.

2. Declare a function to be used to manage the surplus amount, i.e. the difference between the token balance of the protocol minus the value stored. If called by admin, it can send surplus to the specified address. If called by a random address, it can only send the surplus amount to the admin address.

[smol-ninja]

Should we release this feature in the next lockup release as well the first Flow release?

[PaulRBerg]

Yes we should