Refactoring/executor

[finomayato]

Changes

• Refactored executor. Not, the executor does not require any threads
• Introduced services. This module is going to be a middleware between models and apis. It will keep the logic of execution for example. Try to not use models directly, but always create service for it
• Added tests for execution flow and for executor
• DB operations moved out of executor module
• Changed module structure: renamed backend into core(because everything is a backend except client), removed db module, moved models module into the core
• Simplified code related to session creation
• Removed GET /api/v1/jobs/${job_id}/runs endpoint. Did not found where it is used
• Removed PUT /jobs/<job_id>. Was created as a generic method for toggling schedule. Now not in use

TODOs

• [x] Remove all models imports from apis
• [ ] Refactor job_executor structure and merge in core
• [ ] Rename env variables related to the DB
• [ ] Create separate module for standalone executions
• [ ] Populate database with default entrypoint and requirements parameters
• [ ] Add error flag on logs with exceptions

[finomayato]

Note: we check auth headers only for CLI requests

[finomayato]

Right now we raise custom exception from services. Some of these exceptions should be piped into the tasks logs