bench get-app - support for mono repos and "App Store" like repos

[varun-krishnamurthy]

Issue: Feature Request

Is your feature request related to a problem? Please describe.
Currently, bench get-app assumes that there is only one app in the github repo. This would mean:

1. Org developing multiple apps needs to have indivdual repos, CI processes etc, leading to additional over head
2. In the container based deployment, CI process needs to overlay every app one at a time resulting in huge image.

Describe the solution you'd like

bench get-app allows the following:

1. --context-dir parameter - This helps devs to fetch the application under a particular folder of the mono repo.
2. --import-all - This helps devs to import all the apps inside a mono repo and install them.

Describe alternatives you've considered
Create individual repos for each app.

Additional context
This feature will let orgs to create a Linux Package repo/App store like environment wherein clients get access to the mono repo and install the required app provided by the vendor.