Implement plugins

[r4victor]

Closes #2576

The PR adds support for server-side dstack plugins. Plugins are enabled via server/config.yml:

plugins:
  - my_dstack_plugin
  - some_other_plugin
projects:
- name: main

To implement a plugin, plugin authors create a Python package with a dstack.plugins.Plugin subclass and export it as a package entrypoint:

[project.entry-points."dstack.plugins"]
my_dstack_plugin = "my_dstack_plugin:MyPlugin"

Currently, the only plugins functionality is ApplyPolicy that change apply specs (run/fleet/volume/gateway).

Notes:

• ApplyPolicy.on_apply() will usually be called twice: on getting a plan and on apply. So if on_apply() is non-deterministic, the spec displayed in the plan can be different from the actual applied spec. Discussed with @peterschmidt85 that this is a desired behavior as otherwise plugin authors would need to double-check the modified spec.

A plugin guide is TBD.