SFC: ERC 20 bridge: all different ERC 20 configurations on the same bridge

[drinkcoffee]

The ERC 20 bridge Solidity code (https://github.com/ConsenSys/gpact/tree/main/application/nonatomic-appcontracts/erc20bridge/src/main/solidity) at present allows multiple ERC 20 token types to be transferred across the one bridge. However, a bridge contract is either deployed as a Mass Conservation bridge, or a Minting / Burning bridge. Each token is likely to need a different configuration.

For example:
Blockchain A. Blockchain B. Blockchain C
Token A Mass C. Mass C. Mass C.
Token B. Mass C. Mint Burn
Token C. Mint Burn. Mass C. Mass C.

To support this variety of ERC 20 token configurations, the bridge needs to have a configuration item when a token contract is added, indicating what type the token contract is. That is, the configuration should specify how the bridge should use the token contract: transferFrom or mint.

[drinkcoffee]

A fix for this in on erc20bridge-unify branch

[drinkcoffee]

Pull request: #35