Ability to update and delete records

[zakum1]

I have created a fork with the ability to update and delete records.

https://github.com/ztech-company/strapi-plugin-import-export-content/tree/update-delete

To isolate this behaviour (in case users want to keep the existing behavior), a checkbox "Allow Update and Delete" is added to the upload screen.

If the checkbox is selected then:

1. For each record, a check is made whether it exists (based on id) and if so it is updated rather than created
2. If the record has an attribute operation_delete then the record is delete (based on id). Records with this attribute still need to be valid records, because they are validated as part of the field mapping (I use this for bulk deleting, although the way it is done is not ideal).

Some of this design could possibly be improved, but am keen to get feedback on it and to get your thoughts on whether you see benefit in merging this into your repo?

[AdamQuadmon]

Ciao @zakum1 thanks for your code
I'm testing your branch so I opened a PR about the checkbox rendering

[ishallwin20]

@zakum1 : Have you implemented it? PR just has UI changes required for this.