Update signature for tab_spanners to indicate label can be md(), html() result

[machow]

https://github.com/posit-dev/great-tables/pull/345 supported having more than a single spanner. As a part of that, we added some quick logic for labels that are md() and html() result objects.

Let's be sure to...

• update the tab_spanners label annotation to reflect these objects are possible
• update the logic to use isinstance checks rather then hasattr.

(Alternatively we could use structural subtyping and support anything with a specific attribute, but I'd lock it down for now!)