Allow exposed APIs to indicate items are deprecated.

[mhammond]

It would seem ideal if we can mark an item (for a vague definition of "item") in the UDL as deprecated (eg, a [Deprecated] attribute) . The foreign bindings could then generate whatever annotations make sense in that language (eg, @deprecated for Python) so that the final consumers of the API can know about the deprecation.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-156

[yukibtc]

Any plan to support this with proc-macros?

[mhammond]

You can imagine metadata holding some named attributes for items - docstrings could also be carried here for example.

[engali94]

on it @mhammond thank you!