Ability to store metadata about functions

would be very handy to have the ability to store metadata about functions

analogous to metadata for topics

https://github.com/apache/pulsar/issues/12629

advantages (same as for topics):

> Introduce the ability to store metadata about topics.
> This would be very useful as with metadata you could add labels and other
> pieces of information that would allow defining the purpose of a topic,
> custom application-level properties.
> This feature will allow application-level diagnostic tools and maintenance
> tools to not need external databases to store such metadata.
>
> Imagine that we could add a simple key value map (String keys and String
> values) to the topic.
> These metadata could be set during topic creation and also updated.
>

The issue names no files, tests, or function-management entry point; start by reading the linked topic-metadata issue (#12629) and the function APIs. Clarify the metadata shape and creation, update, and retrieval behavior, then add tests that demonstrate the agreed scope.

Written by the indexing model from the issue text.