Use Dash's create_callback_id to get the callback id

[RenaudLN]

The current public_callback decorator wasn't working well in some cases, including when using dash-extension's DashProxy.

This PR leverages Dash's internal create_callback_id to retrieve the callback id to be marked as public.

Also copied test updated from #162 as this PR supersedes the fix in there.

[RenaudLN]

@alexcjohnson if you could have a quick look at this one as well 🙏

[ndrezn]

Hey Renaud, I'll have the team take a look as this. For future reference feel free to tag myself or @gvwilson if you have any questions on GH.

That being said as this adds new functionality to the package I am not immediately sure whether we will accept this PR.

[RenaudLN]

Hey @ndrezn this isn't new functionality, it's a fix. The public_callback doesn't work in a number of cases, including when some clientside callbacks have been defined prior.

[antonymilne]

Just wanted to add that I also came across this bug completely independently of @RenaudLN.