[fabric-samples] Add ERC20 fungible token sample

Add a sample of how to use a fungible token in Chaincode.
This Chaincode is a sample implementation of [ERC20](https://eips.ethereum.org/EIPS/eip-20),
which is a standard interface of tokens in Ethereum.

ERC20 provides basic functionality to transfer tokens, as well as allow tokens
to be approved so they can be spent by another on-chain third party.
Most of major tokens on the Ethereum are ERC20 compliant.

The functionality defined by ERC20 is also useful for Chaincode in Hyperledger
Fabric. Since ERC20 is a specification of token interfaces, it can be implemented
by any blockchain software. Thie sample shows the ERC20 for Hyperledger Fabric
so that we can use the major token protocol.