Update guidance: published interfaces can also evolve via default method implementations

[onbuyuka]

Summary

Update the guidance page microsoft/knowledge/interfaces/extend-published-interfaces-dont-edit-them.md to reflect that AL interfaces now support default implementation methods, which can provide a non-breaking way to evolve published interfaces.

Problem

The current page recommends extending a published interface via a new interface rather than adding methods directly, because adding a method changes the contract and breaks existing implementers.

That guidance misses a newer AL capability: interface methods can now have default implementations. Adding a new method with a default implementation can allow existing implementers to remain compatible.

Requested changes

• Update the guidance to acknowledge default interface method implementations as an additional evolution strategy.
• Clarify when to prefer:
  • creating a new interface that extends the published interface, versus
  • adding a default method implementation to the existing interface.
• Document tradeoffs and caveats for each approach (readability, versioning clarity, compatibility expectations, and long-term maintenance).
• Update or add samples to show the default-method pattern alongside the existing extend-interface pattern.

Acceptance criteria

• The page explicitly mentions default implementation methods as a non-breaking option.
• Best-practice section explains decision criteria between extends and default methods.
• Samples are updated/added to demonstrate the new guidance.
• Existing guidance intent (avoiding breaking changes for implementers) remains clear.

Reference

• Page: microsoft/knowledge/interfaces/extend-published-interfaces-dont-edit-them.md
• URL: https://github.com/microsoft/BCQuality/blob/1a5afdc0ebb6eb3fde1c3c69a13f235275cf4549/microsoft/knowledge/interfaces/extend-published-interfaces-dont-edit-them.md