Feat/payload validation

[Khaledgarbaya]

This PR adds payload validation for the entities to be imported based on a JSON schema.
If the one of the entity's payload is invalid it will be removed from the array and it a nice warning will be logged telling the user which field is missing and the entity id if possible.

Why?

• centralize all checks in one pass.

This is still a WIP PR I would like to get your inputs on it and how strict should we go in the schema validation.

[zcei]

telling the user which field is missing

This would involve a lot of dynamic schema building, as you need to compile a new JSON schema per Content Type to check an Entry against it.

I would probably only check for keys that are required to pass our Management API, e.g. a field in a ContentType needs the type property.

While building the migration tooling we used joi and even there hit some limitations, so only JSON Schema would be very ambitious if you plan to validate Entries.

I recently found superstruct, haven't used it yet, but might a good fit.

[Khaledgarbaya]

@zcei I don't want to go crazy with the validation, I will just make sure the keys expected by the transformation pass and by the management api.
stuff like assets without file field etc

[axe312ger]

I like this idea a lot :)

Superstruct looks pretty nice, also :)

Lets discuss monday :)

[axe312ger]

@Khaledgarbaya I guess we can close this in favour of contentful/contentful-import#81?