Support directory of files

[AlbertDeFusco]

Allow endpoints to be gathered from any discovered .py and .ipynb files in a chosen directory.

See https://github.com/holoviz/panel/blob/4be32f416345f599b866e61241454847c6adce9b/panel/io/rest.py#L82

[philippjfr]

I'd be happy to see a function in handlers that returns the tranquilized functions given a filename, not sure it needs to handle a directory or multiple files. I'd probably also make handlers explicitly declare the filetypes they support so you can just iterate over all BaseHandler subclasses to determine which applies for a particular filetype.

[AlbertDeFusco]

by "returns the tranquilized functions" you'd want the output of make_app() as it is today? This returns an initialized restx Api object. https://flask-restx.readthedocs.io/en/latest/api.html

[philippjfr]

That would be okay too, I was thinking it would just return the full set of tranquilized functions discovered in the file(s) which could then be passed to make_app, but I trust your judgement here.