Create command to show available Schedulers

[Ahmad-Wahid]

This PR builds upon the previous work done in PR #686, which introduced the command flexmeasures show reporters. Now, we extend this functionality to include the Scheduler class as well. The new command, flexmeasures show schedulers, allows users to view a comprehensive list of all available schedulers within flexmeasures.

[Flix6x]

Let's refactor to prevent more or less duplicate code.

I think the common term we use for schedulers, forecasters and reporters is data generators, so the util function could use that term.

[Ahmad-Wahid]

I have created a generic function list_items which is called by the following commands.

Executing flexmeasures show schedulers would display the relevant information.

name                version  author    module
----------------  ---------  --------  -----------------------------------------
StorageScheduler          1  Seita     flexmeasures.data.models.planning.storage

and flexmeasures show reporters would show

name              version  author    module
--------------  ---------  --------  --------------------------------------------------
PandasReporter          1            flexmeasures.data.models.reporting.pandas_reporter

[Flix6x]

Nice work refactoring, @Ahmad-Wahid!