Add EntityExtractor as a requirement for EntitySynonymMapper

[AMR-KELEG]

Closes #6198

Proposed changes:

• Add EntityExtractor as a requirement for EntitySynonymMapper in a pipeline

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

Comment:
I failed to test the PR locally (I have missed up with my clone and I haven't been able to build Rasa from source yet).
I would really appreciate if you can test the edit.
The change is intuitive yet it needs to be tested.

[sara-tagger]

Thanks for submitting a pull request 🚀 @akelad will take a look at it as soon as possible ✨

[akelad]

@tabergma do you know why that wasn't there in the first place?

[tabergma]

@akelad No idea. We changed the way how to define those required components a couple of month ago. It might be that is was not possible beforehand and we forgot to add it once we changed. But this is definitely a good requirement that we should add.

[akelad]

agreed, was just checking whether there was a reason we omitted it 👌

[akelad]

thanks for the quick fix @AMR-KELEG! looks good + also works as intended.

Could you add a changelog entry about this bug fix as well please?

[AMR-KELEG]

Thanks @akelad for verifying that the change works.
I will try to figure out how to properly build Rasa from source soon.
I have added an entry in the changelog as requested.

On the other hand, Should also the doc rst file be modified?

[akelad]

good find, yes that should also be changed there!

[akelad]

Thanks a lot for this great contribution!!