Channels list endpoint - return pending channels

[rakanalh]

Abstract

Follow up to #4621, lightning has a feature where channels whose creation transactions have not been mined are displayed with a "pending" status.

Motivation

Having better UX in Raiden, means that the channels which were requested to be created but their creation transaction is not yet mind, should still be displayed / returned via the /channels endpoint.

Specification

• Acquire an architectural decision on whether Raiden should implement this. If so, how?
• More specs to be added later.

Backwards Compatibility

New feature / backwards in-compatible.

[hackaugusto]

related: #1621