refactor: configurable posting date for Exc Gain / Loss journal (backport #44790)

[mergify]

Issue

Consider a situation were the payment and invoice are both in locked period. If it is a forex transaction and if they incur any exchange gain / loss, system tries to create a Journal in the payment date to capture that gain / loss. This will fail due to the period being locked.

| Oct (Locked) | Nov (Locked) | Dec            |
|--------------+--------------+----------------|
| Payment      | Invoice      | Reconciliation |

Configurable Exchange Gain / Loss posting date (Only applies for Normal Payments)

todo:

• patch to set default config value
• Test cases
• Support Gain / Loss JE posting date configuration in Semi-Auto Reconciliation tool

ref: https://support.frappe.io/helpdesk/tickets/28039

---

This is an automatic backport of pull request #44790 done by Mergify.

[mergify]

Cherry-pick of a717188 has failed:

On branch mergify/bp/version-15-hotfix/pr-44790
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You are currently cherry-picking commit a71718883e.
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Changes to be committed:
	modified:   erpnext/accounts/doctype/process_payment_reconciliation_log_allocations/process_payment_reconciliation_log_allocations.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/accounts/doctype/process_payment_reconciliation_log_allocations/process_payment_reconciliation_log_allocations.json

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