Fix form multiselect request validation

[PierrickBrun]

When using a FORM Datasource, if a multiselect field is present, quart-schema will fail to validate the data.

This is because MultiDict.to_dict() returns only the first occurrence of each key by default.

By using flat=True this PR fixes this.

ref: https://stackoverflow.com/questions/51992901/capture-values-from-multiple-select-form-and-post-via-flask

https://werkzeug.palletsprojects.com/en/3.0.x/datastructures/#werkzeug.datastructures.MultiDict.to_dict

[PierrickBrun]

Hi @pgjones , any opinion on this PR ? thanks

[pgjones]

Thanks