Make 'Mutation' configs optional

[CHE1RON]

This is the same as for Query etc, since otherwise users always have to specify key (which is not the case when specifying Query options, and users should not have to):

Also see the docs where key itself is optional.

[changeset-bot]

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[stijnvanhulle]

@all-contributors please add @CHE1RON for code

[allcontributors]

@stijnvanhulle

I've put up a pull request to add @CHE1RON! 🎉

[CHE1RON]

Thanks very much! 😊