SEP-41: permit additional event topics

[leighmcculloch]

What

Clarify that SEP-41 token implementers may emit additional topics beyond those defined, and recommend that any such extension be standardized in its own SEP for interoperability.

Why

The spec was silent on whether implementers could extend events with extra topics, leaving consumers uncertain about what to expect and discouraging interoperable extensions. Whilst I always took this to be an assumption in the ecosystem for any contract SEP, it's become apparent in conversations that was not their assumption. SEP-41 is an important enough standard this should be documented explicitly.

Note that this is not changed behaviour, but rather capturing existing behaviour already playing out in the ecosystem.

Discussion

• Formalize event topic extensions behavior in SEP-41 documentation #1995

[github-actions]

👋 Thanks for your contribution!

This PR proposes changes to the following existing SEP(s):

• ecosystem/sep-0041.md — author(s): @jonjove, @sisuresh, @chowbao — existing discussion

Per the SEP Contribution Process, changes to an existing SEP should start as a discussion, and should be advocated with the SEP's author(s) — the author maintains the proposal, so engaging them is an important part of getting a change adopted. If you haven't already, please raise your proposed change on the SEP's existing discussion linked above, or start one on the GitHub discussion forum.

I have marked this PR as a draft for now. Once the change has been discussed and is ready for a maintainer to review and merge, mark it as ready for review.

Thanks for helping improve the Stellar ecosystem! 🚀