Formalize event topic extensions behavior in SEP-41 documentation

[leighmcculloch]

Hi all, I think we should formalize event topic extensions in SEP-41.

The Stellar Asset Contract emits SEP-41 events with an additional topic, the sep0011_asset field. It has been assumed by a few of us for sometime that any contract could extend the topics of a standard, but it's become apparent through conversations that this assumption is not obvious.

Even if this was, is, or could become an implicit understanding with any SEP contract's events, SEP-41 is so critical as a standard I think in a similar thinking as #1919 we should be explicit and remove this assumption from SEP-41.

I suggest adding a short paragraph to capture it:

Implementers may emit additional topics beyond those defined for each event.
To improve interoperability, it is recommended to do so as part of a SEP that
documents the behavior and when contracts should emit those additional topics.

PR: #1994

[mootz12]

Agree that if this is already occurring, we need to document the behavior.

Did we consider just specifying that the standard supports including an additional topic for the SEP-0011 asset string, instead of anything?

I'm not sure it really matters, but it might be good to get some downstream opinions from people that consume this type of data, and if opening up topics to be "at least these fields" is fine, cc @orbitlens or @itsfederico

[itsfederico]

From our indexer side, "at least these fields" is fine. Matching is done on the standard topics as a prefix, so extra trailing topics never break anything. The SAC asset topic has never been a problem for us. Restricting the spec to just the SEP-0011 topic wouldn't simplify anything on the consumer end since the handling logic is the same either way, so the generic wording is fine on our side.

[orbitlens]

No objections. It wouldn't affect our pipeline.