Introduce the DEM API

[janosmurai]

Type Of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Any modification in the test cases
• Any modification in the documentation

Checklist:

• I have read and followed the contribution guideline.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have made corresponding changes to the documentation.

Related Issue

Closing:
DEM-321

Description

Introduced the FastAPI based DEM API.
As the first iteration it can handle only one command, the run.
Added advanced task definition.
To be able to implement these modifications major restructuring was necessary.

How Has This Been Tested?

Tested on Ubuntu 22.04, with two test containers. The first was started as a
DEM task, the second has been initiated from the first one using the new API.