Refactor backend

[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

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

  • Backend infrastructure not dynamic
  • Task, TaskArgs, TaskResults all have to be specified in oqd-core
  • TaskResults is a pydantic datamodel

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

  • Remove Task wrapper object (not necessary)
  • Remove TaskArgs, TaskResult, these are now handled internally in the backend definition.
  • Added BackendRegistry this enables dynamic definition of Backends that are then automatically added to the BackendRegistry.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  All old code defining Backend requires updating to the new structure.

• Other information: