[Subscription Billing] Fix deferral creation, release, and credit memo handling

[miljance]

Summary

This pull request introduces a new data upgrade step to populate posting group fields in contract deferral records, along with some code clean-up and improvements/fixes across related modules.

1. Extensibility: Five core deferral objects need integration points (events) for ISV extensions to customize behavior during deferral creation and release processes.

2. Missing Posting Groups in Deferrals: Contract deferrals cannot be released when the original posted documents are unavailable (e.g., in data migration scenarios), because posting group information is retrieved from posted invoice/credit memo lines instead of being stored in the deferral entries.

3. Zero-Amount Deferrals: The system creates unnecessary deferral entries and G/L postings for contract lines with 0.00 amounts, leading to excessive ledger entries with no financial impact.

4. Inconsistent Partial Period Handling: Vendor contract deferrals handle partial billing periods inconsistently—some invoices are deferred based on full months while others are calculated daily, causing incorrect revenue recognition.

5. Credit Memo Deferral Gap: Credit memos with amounts exceeding the original invoice amount do not generate corresponding deferral entries, resulting in incomplete revenue recognition.

6. Double Deferral Generation: When both contract-based deferrals and standard deferral codes are used, deferrals are incorrectly created in both systems simultaneously.

Work Item(s)

Fixes #6281

[vkozlov-sit]

Hi Miljan, please check the comments, rather optimization related remarks.