As a Builder, I want to send signed requests to an intermediate Backend before persisting via a Request Node

[MantisClone]

Resolves #1380

Problem

Today, the Request Network SDK createRequest() function performs all the steps to create a request. Notably it:

1. Signs the request
2. Sends the request to the request node to be persisted.

However, some builders would prefer to send the signed request to an intermediate backend that they control before sending it to the Request Node to be persisted.

Motivation

• A couple of our Builders have requested this feature
• We think that this would allow the intermediate backend to handle confirmation and retries instead of forcing the frontend to handle them.
• We think that some builders prefer the frontend to communicate with only 1 backend.