feat: create the CLI command flexmeasures show reporters

[victorgarcia98]

An interesting feature after the flexmeasures add report is the possibility to show all the available reporters. This PR creates a new command that list all the reporter classes that can be used: all the classes in flexmeasures.data.models.reporting and in the main file of every plugin (init.py) that subclass Reporter.

Usage

$ flexmeasures show reporters

Output example

Reporters:

name              version  author
--------------  ---------  --------
PandasReporter          1  None

[Flix6x]

Nice. Looking forward to having this for schedulers and forecasters, too.

Just two small change requests.

And can you create follow-up issues for schedulers and forecasters?

Schedulers:

• Plugins can already register custom schedulers (see documentation). The schedulers currently have an implicit registry as "custom-scheduler" sensor attributes (see flexmeasures.data.services.scheduling.find_scheduler_class), and this registry should be made explicit, following the example set by the reporter registry in Creation of Reporter class #641 and feat: create the CLI command flexmeasures show reporters #686.

Forecasters:

• Forecasters cannot be registered by plugins yet, but we want to follow the same concepts as for reporters and schedulers.

[Flix6x]

I'd put the module last, not first. The name is most likely the bit of information that the user is looking for, so should be first.

Still missing:

• changelog entry
• new entry in documentation/cli/commands.rst