"Client of client" contracts

Modify client contract to be able to accessed and used directly by an end user that did not author the contract. In other words, build a "client of client" contract that allows multiple clients to be able to interact with deal making primitives at the same time through one contract.

This is required to build end user facing dapps that interact with one client contract to make deals on behalf of many users.

Currently the dapp frontend requires the user to deploy their own contract: https://github.com/filecoin-project/fvm-starter-kit-deal-making/issues/2

For a dapp to eventually get built off this client contract, we need to:
- Ensure that an end user can interact with a client contract that they did not deploy (eg figure out what is happening here: https://github.com/filecoin-project/fvm-starter-kit-deal-making/issues/2)
- For paid deals, ensure that the contract does the proper accounting to keep the escrow flows to the market actor "whole" to the user. In other words, if user A adds 10 FIL to the market actor, and user B adds 2 FIL to the market actor, we need to ensure the 12 FIL are accountable to users A and B. Users A and B should be able to add to / withdraw from their portion of the escrow and be "made whole" whenever possible.