Use a runtime instance in the SDK's EthereumClient

[jvff]

Motivation

The EthereumClient currently is hard-coded to use the low-level WIT interface to perform HTTP requests. This is different from the rest of the SDK which uses the runtime types to perform system API calls. However, it's currently not possible to use a single runtime type, and the EthereumClient had to be split in two types in PR so that it could use either the contract or the service's system API.

Proposal

After #1977 has been implemented, the two types should be reunited into a single type, and the base/common runtime should be used.

[Gnomon123321]

good!