feat/integration log references

[blaggacao]

• feat: link ecommerce integration log with payment session log
• feat: add reference doc to ecommerce integration log

Context

• Frappe may act as "custom payment method"
• The special checkout page captures the order data
• Then it either or issues (background) synchronization
• It then redirects to the payment session
• The EIL becomes the central integration document for in-flight orders
  • Therefore the payment status and PSL is replicated onto it to quickly assess the overall status of such checkout
• The payment session runs a success hook on its refdoc (= the EIL)
  • Therefore hold a reference to the refdoc of the EIL
  • This allows charge success hooks to affect the business flow of the actual target refdoc (i.e. Sales Order), assuming that the synchronization had been successful and that it had written back the reference of the target doc onto the EIL

This is used by a WIP Ecwid integration, but which is not yet quite ready to PR.

[blaggacao]

This requires frappe/payments#53

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