EA Exports

MERGE REQUEST

Overview

In order to construct the withdrawal message and perform decent error handling,
some of the structs and errors need to be moved from the account manager over
to the modules package.

I've added them in payment.go. In another MR, this file is going to be extended
with all payment request/response objects.

Note: I have not yet moved all of the errors over to the modules package. Currently,
the renter does not use a single one of these errors, I think we can gradually
move errors over to the modules package when there's a need for it in the renter.

Example for Visual changes (ie Screenshot)

N/A

Issues Closed

Closes #3785 (closed) (TODO: verify - not 100% sure)

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated