Allow gcclient to connect to chaincode from GC gateway URL

[dzikowski]

Currently gcclient allows to connect to GC gateway with forApiConfig method, which requires the following parameters (assuming #231 is resolved):

export interface RestApiClientConfig {
  apiUrl: string;
  userId?: string;
  userSecret?: string;
  configPath: string;
}

We would like to allow an additional way of connecting to the contract, by contract URL. We should add gcclient.forContractUrl(url) method which accepts just a contract URL, like: https://gateway.stage.galachain.com/api/hackathon03/gc-8c757ffb9ac963b04e45b5e6e8a298992c530c23-GalaChainToken. Then it would use a similar flow as RestApiClient, just with no need of fetching contract API.

Probably the best way to approach is to:

1. add gcclient.forContractUrl(url) method
2. copy RestApiClientBuilder and remove non relevant code (duplication is good here)
3. copy RestApiClient and remove non relevant code (also duplication is good)

Then we can use this way of connecting to the contract with @gala-chain/connect