feat: add new payment processing hooks

[blaggacao]

Context

frappe/payments#53 implements a new version of payment controllers which are more suitable for webshop checkouts and have a better frontend user flow.

The new version comes with a well-defined assortment of ref doc hooks for post-processing a payment flow within the business logic.

Proposal

• This PR adds the new post-processing hooks
• It also marks the previous ones as "Legacy Gateway"

no-docs

[stale]

This pull request has been automatically marked as stale 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.

[stale]

This pull request has been automatically marked as stale 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]

Please exempt from closing.

[blaggacao]

@ankush can you excempt this from the bot, please?

[blaggacao]

Why is the stalebot not respecting comments to understand that this is still rightfully pending? @ankush or @akhilnarang could you please do something about this issue, it's increasingly annoying and an absolutly horrible contributor experience.