Handle plugin dependency managment

[pauldix]

Plugins should be able to use third-party dependencies. Ideally, when installing a plugin, the server will download the dependencies and make them available. We'll also want a way to tell the server to load dependencies in advance of specifying a plugin explicitly.

I anticipate that most plugins will be a single file script of less than a few hundred lines, so I think using in-file dependencies in a comment block at the top. We'll use the uv syntax with the dependencies in a comment block at the top:

# requires-dist: requests>=2.31.0
# requires-dist: pandas>=2.2.0

When the plugin is created in the server, or the test plugin API is called, it should check for dependencies and install them.

If possible, we'll want a virtual environment per plugin, but that doesn't have to exist on this first pass.