Include new task metadata in manifest

[tcompa]

The most-recent manifest schema is at https://github.com/fractal-analytics-platform/fractal-server/blob/main/fractal_server/app/schemas/v2/manifest.py.

Notable changes:

1. Manifest has a new top-level attribute authors: Optional[str] = None (note: it's a single string, not an array - this was done to simplify queries like "authors contains xyz)
2. Each task has three new attributes:
   • category: Optional[str] = None
   • modality: Optional[str] = None
   • tags: list[str] = Field(default_factory=list)
3. There is a new constraint that no two tasks of the same package can have the same name

To do:

• Update what is needed in https://github.com/fractal-analytics-platform/fractal-tasks-core/tree/main/fractal_tasks_core/dev
• Actually fill the values for the top-level authors and for all individual tasks
• Document the "typical" conventions somewhere (fractal-tasks-core docs? or somewhere in https://fractal-analytics-platform.github.io/?)

[tcompa]

Update what is needed in https://github.com/fractal-analytics-platform/fractal-tasks-core/tree/main/fractal_tasks_core/dev

This is mostly covered in #855, up to a decision of where the top-level information about authors should be stored.
I've started by placing it into the create_manifest.py script (which can also be automated, within fractal-tasks-template, based on the copier answers), but we can also consider other options (e.g. expanding the task_list.py module, to also host some top-level information).

The other two points (actually filling the metadata of fractal-tasks-core, and documenting the conventions) remain open.