Add virtual development environments to the `ubuntu-20.04-upgrade` branch

[WadeBarnes]

Add Visual Studio Code and Gitpod virtual development environments to the indy-plenum project's ubuntu-20.04-upgrade branch. Ubuntu 16.04 is not supported as a DevContainer so we're not going to go through the effort of trying to support virtual development on 16.04.

This has already been done for hyperledger/indy-node and can be used as a reference:

• https://github.com/hyperledger/indy-node/pull/1718
• https://github.com/hyperledger/indy-node/pull/1722

Outstanding issues from the above:

• https://github.com/hyperledger/indy-node/issues/1728
• https://github.com/hyperledger/indy-node/issues/1729

[WadeBarnes]

cc @pSchlarb, @udosson, @swcurran, @lynnbendixsen

[WadeBarnes]

done