Allow addons to extend the contents of the public directory with their own public folder

[JeffersonBledsoe]

Is your feature request related to a problem? Please describe.

Currently, for an add-on to add static files, an express middleware needs to be used. While this isn't too much of a problem, it would be nice if add-on development felt the same as frontend development and placing a file in the public directory made it available.

Describe the solution you'd like

Being able to add a file to a public directory inside of an add-on and it be available in the site.

[tiberiuichim]

Based on discussion with Jefferson, we propose the following behavior:

• an addon can declare a staticResourceOverlay which points to a folder similar to the public folder of a project.
• these folders are virtually "overlaid" one on top of the other, in the addon declaration order, so that the static resources can be included or overriden

CC @JeffersonBledsoe to confirm this proposal.

[JeffersonBledsoe]

Looks good!

[wesleybl]

@tiberiuichim I think it's a good idea to overwrite or add public folder URLs. But would the URLs be saved in memory, so you don't have to read the disk with each request?

[davisagli]

A way to do this was added in #6072