NetworkController: Add RPC service class

[mcmire]

Explanation

This commit adds an RPC service class that will ultimately be used inside of middleware to make JSON-RPC requests to an RPC endpoint. It makes uses of createServicePolicy, added in a previous commit, to encapsulate the request code using the retry and circuit breaker policies. As this service class is designed to replace large parts of the fetch and Infura middleware, it customizes the service policy so that the request will be retried only when the network is perceived to be "down". This occurs when:

• The fetch call throws a "Failed to fetch" error (or something similar to it; see code for the full list of variations)
• The fetch call throws an ETIMEDOUT or ECONNRESET error
• The response status is 503 or 504
• The response body is invalid JSON

In contrast, the network is not perceived to be "down" if:

• The fetch call throws an unexpected error (e.g. if the request options are invalid)
• The response status is not 2xx, but is also not 503 or 504
• The response body is an unsuccessful JSON-RPC response
• The response body is a successful, but empty, JSON-RPC response

References

Closes #4990.

Changelog

(N/A; internal change only)

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes