Support ETHEREUM_PROXY_CONTRACT payment network in request-client.js

[MantisClone]

Problem

The EthereumProxy contract (ETHEREUM_PROXY_CONTRACT payment network) should be supported by request-client.js.

It is already supported by the payment-processor package.

Solution

Implementation should be fairly quick, just need to wire up a few function calls.

Priority

Pending #1143 to learn if improving ETHEREUM_PROXY_CONTRACT support is worth the effort.

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