Refactor providers, add custom docker images support

[Egor-S]

Closes #540
Closes #550

• Replace providers with job configurators
• Validate and parse configurations and profiles with pydantic models
• Allow to configure custom docker image

TODO

• Remove providers completely
• Add entry point to a configuration
• Validate custom docker images environment on the start (sshd, pip)
• Test & debug edge cases
• Handle validation errors
• Print help for loaded configuration
• Generate JSON schemas from pydantic models
• Update docs/reference