Etherscan upgrades

[martinheidegger]

This PR combines a bunch of improvements we had to do for etherscan to work better.

• 34a07f0 - feat: adding support for api-keys
  Etherscan requires API-keys to be used for a while now. With this commit adds a apiKey property to etherscan provider.
• 75c4185 - feat: allowing to specify networks using common network specifiers
  The networks, specified in other parts of the code, use things like mainnet, goerli etc. as specifier. these are available at etherscan through a api-<network> subdomains. This commit should make it easier to specify the network correctly.
• 9e09aa0 - fix: allowing any kind of statusCode
  Before this commit only 403 error were handled by the subprovider. With this commit any unexpected status code is treated by errors.
• 69615bd - fix: returning the error itself if the message isn't given.
  There is a chance that errors are returned without message, in this case the callback is returned with null, indicating no error occurred. With this commit it is made sure that an error is always returned.
• d5650e2 - fix: de-referencing interval if no request is pending.
  Before this commit an interval was running by merely instantiating Etherscan. Now it runs only if there is actually work that may be done.
• e8d91c7 - fix: only use defined properties during transaction listening.
  We use configuration properties to fill the requests to etherscan. This commit allows to specify one as "undefined" and not break the request.
• cb61e5b - fix: making sure that callbacks are always called with error instances
  Callback API's usually return error instances in the first argument. With this commit every response is guaranteed to be an error instance
• 40105e8 - feat: adding support for gas estimate through etherscan
  Etherscan has a convenient API to calculate the gas required for an operation. With this commit it can be used.

[mcmire]

Thank you for this pull request, but we are actively phasing out this package in favor of json-rpc-engine and its related packages, so I'm going to close this. As for the changes here, we no longer have any Etherscan-specific middleware. Depending on your use case, you may wish to make a new package which recreates this behavior and plugs into our new middleware pipeline.