Directive chooser middleware

[ivelten]

With this Pull Request, I'm proposing an operation planning middleware that changes the current operation based on directives present in the query. The user can choose directives with a chooser function(Directive -> Directive option) by placing it in the Metadata object of executor.AsyncExecute method. If the middleware is present in the pipeline, it will look for this chooser function and apply it to the operation.

• [x] Create middleware initial version
• [x] Unit tests
• [x] Documentation

[xperiandri]

And this