[BuildStream] Add setting to disallow insecure transports

[Frazer Clews <frazer clews codethink co uk>]

Hi

For this issue I have come up with some solutions

Some being similar to the acceptance criteria by adding a setting to 
project.refs, although not sure if project.conf is more appropriate. Or 
by setting an optional flag to either toggle or set the option to only 
allow secure sources, personally prefer the latter as a toggle could get 
confusing going from project to project.

I have been experimenting where to put in the check for the type of 
protocol being used and stop it, but only got it properly working so far 
for ftp in downloadablefilesource.py and feels kind of hacky, so will 
look into a better solution, but happy for suggestions

What do you think so far?

Thanks

Frazer

https://gitlab.com/BuildStream/buildstream/-/issues/408