[yhteoh]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  refactor, feature

• What is the current behavior? (You can also link to an open issue here)

• Non flexible backend infrastructure
• Task and TaskArgs must be defined in oqd-core
• No registry for registering backends to

• What is the new behavior (if this is a feature change)?

• Added registry for backends (future support for automated generation of job submission API)
• Added flexible definition of program and options to the run function that need not be in the oqd-core

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  previously implemented backends need to be refactored to match the new backend interface

• Other information: