feat: multiple network/bridge support ?

[paouvrard]

Moving the discussion here from https://github.com/near/rainbow-bridge-client/pull/4#pullrequestreview-604714953 and https://github.com/near/rainbow-bridge-client/pull/4#issuecomment-791079039

Dapps may want to connect to different bridges simultaneously and record bridge transfers of different bridges on the same URL. The current lib design assumes that dapps like https://github.com/near/rainbow-bridge-frontend will be connecting to a single bridge per URL (rinkeby.bridge.near.org for example) and uses process.env to access bridge contract addresses (process.env.ethLockerAddress for example). Transfers recorded in localStorage do not contain information about token connectors, bridges or networks.