Migrate Python attribute comments to attribute docstrings

[r4victor]

For historical reasons, dstack Python codebase uses comments before attributes to document the attributes (unless there is a more appropriate place like Field(description="...")). These comments have no special meaning to Python tooling, are not recognized by the IDEs, so you don't see any help when hover over an attribute in IDE.

Python has “attribute docstrings” convention as described in PEP 257:

String literals occurring immediately after a simple assignment at the top level of a module, class, or init method are called “attribute docstrings”.

These are recognized by Python tooling and the descriptions are visible in IDEs. Compare docstrings vs comments:

I suggest we migrate all comments attribute descriptions to attribute docstrings. There is a readability difference in that docstrings go after the attribute but that's Python tooling convention and we won't change it now.