refactor: hide some payments methods, replace a class to namespace

[knst]

Issue being fixed or feature implemented

Here's TODO that seems out-dated

/// TODO: all 4 functions do not belong here really, they should be refactored/moved somewhere (main.cpp ?)

This changes are extracted from this PR: #5342

What was done?

This changes hides some methods from global namespace (making local static function), hiding other functions to the namespace

How Has This Been Tested?

Run unit/functional tests

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone

[UdjinM6]

utACK

[knst]

For easier code review you may find git show --color-moved=zebra 21adeea4b85fa47ce728f850be1ad7725181b1e8 useful so far as it shows moved code by different color.

[ogabrielides]

utACK

[PastaPastaPasta]

utACK for squash merge