feat: enable a new payment controller

[blaggacao]

• [ ... ] (5 commits are also part of preparatory PRs)

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• feat: support a new payment controller

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Here's a review branch without the preliminary code-shaping: blaggacao#1

no-docs

[stale]

This pull request has been automatically marked as inactive because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[blaggacao]

@s-aga-r From the standpoint of coordinating this PR in connection with it's dependent frappe/payments#53 this PR is fully backwards compatible and should be merged first in order to enable meaningful test execution on frappe/payments#53, at all.

Nonwithstanding amendments that will be required to this PR upon further insights gainer through future work on frappe/payments#53, this PR can be deemed at least "ready to review", if not "ready to merge (and iterate later)".

I appreciate your (hopeful) help and collaboration. Thank you in advance!

[blaggacao]

@s-aga-r Could you help with a commitment to engage on this one? My situation is that I want to contract with a well renown Frappe Partner for custom development and it's absolutely essential for us to be able to leverage the revised payment integration that this PR prepares on the ERPNext side. I promise, I'll leave you alone with my heavy handed contributions for a while after this. Sounds like a deal? 😃

[s-aga-r]

@ruthra-kumar can you take this up?

[ruthra-kumar]

@ruthra-kumar can you take this up?

As long as the changes are small and easy to reason about, I can help with review.

@blaggacao This PR is too big to work with. I would suggest breaking it into smaller, atomic changes.

[blaggacao]

#42160 is necessary to effectively leverage frappe/frappe#25952 for frappe/payments#53