refactor!: move payment gateways to the payments app

[s-aga-r]

frappe/payments#44

• GoCardless Settings
• GoCardless Mandate
• GoCardless Templates
• Mpesa Settings
• stripe_integration.py
• Dependencies
• Fix Imports
• Fix Unit Tests
• Patch to delete doctypes
• Some warning (?) - not required

[bosue]

Fixes #37004.

[ankush]

nice, finally a clean separation 👍

[codecov]

Codecov Report

Merging #37182 (296b233) into develop (f0859ec) will increase coverage by 0.07%.
Report is 63 commits behind head on develop.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop   #37182      +/-   ##
===========================================
+ Coverage    66.76%   66.83%   +0.07%     
===========================================
  Files          793      785       -8     
  Lines        62423    62041     -382     
===========================================
- Hits         41679    41468     -211     
+ Misses       20744    20573     -171     

Files	Coverage Δ
erpnext/erpnext_integrations/utils.py	0.00% <ø> (-42.23%)	⬇️
...ccounts/doctype/payment_request/payment_request.py	57.09% <0.00%> (-9.90%)	⬇️

... and 19 files with indirect coverage changes

[blaggacao]

Please kindly advise what to do with: 😄

• fix(payments): early error if payment request is invalid #37503
• fix(payments): separate redirect constructor from authorization #37502
• feat(payment): add advance payment status to advance payment doctypes to better track advance payments #37449
• feat(payment): add 'other' payment channel in addition to email & phone #37448
• fix(payments): incoming payment requests aren't supposed to be in 'initiated' state (only outgoing are) #37447

I'm happy to coordinate and re-target.