Ubuntu 20.04: create CI for Ubuntu 20.04

[Toktar]

Create CI for Ubuntu 20.04 for a feature brunch ubuntu-20.04-upgrade

Acceptance criteria:

• All existing tests are launched via GitHub Actions, and they pass (if pass locally)
• All jobs(steps) pass
• Reports from launches are available.

[WadeBarnes]

The initial (16.04) GHA based CI workflow is working here; https://github.com/WadeBarnes/indy-node/actions/runs/1006012413

• Based on the same work done in hyperledger/indy-plenum
• Once complete, this will be integrated into the ubuntu-20.04-upgrade branch.

[WadeBarnes]

PR for initial (16.04) GHA based CI workflow is working here; https://github.com/hyperledger/indy-node/pull/1680

[pSchlarb]

As #1697 is already merged, can this Issue be closed?