[python-package] [dask] add type annotations on dask._HostWorkers

[jameslamb]

Contributes to #3756.
Contributes to #3867.
Split from #5765.

Proposes replacing the use of collections.namedtuple for one internal class in the Dask module with a regular Python class, so that informative type annotations can be added.

This makes it a bit clearer, when reading the code, what the values of that class's attributes should be (unlike namedtuple, which is just a collection of attribute names).

I chose to use a regular Python class instead of dataclasses.dataclass to preserve support for Python 3.6 (#5765 (comment)), and chose to avoid typing.NamedTuple (https://stackoverflow.com/a/50038614/3986677) to reduce the risk of incompatibilities caused by future updates to typing.

[jameslamb]

thanks @jmoralez !

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed.
To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues
including a reference to this.