Ability to specify custom/enterprise pyPI path to install streamlit & other dependencies

[prashao]

The default stlite.html file currently uses https://cdn.jsdelivr.net/npm/@stlite/mountable@0.63.1/build/stlite.js to use the pyPI to install say streamlit & other packages.

Need ability to instead specify custom pyPI path

[whitphx]

Dev note: will add Stlite API to utilize this micropip option:
https://pyodide.org/en/stable/usage/loading-packages.html#installing-wheels-from-different-indexes

[whitphx]

@prashao Or you can directly specify the wheel URL in the requirements tab. Is it enough?
btw we can't change the Streamlit package origin on Stlite because Stlite is using a customized one.