fix(es): make nullable fields undefinable, dissociate Message and Operation

[bodinsamuel]

Changes

Fixes https://linear.app/nango/issue/NAN-2738/set-nullable-fields-as-undefined

• Elasticsearch: make nullable fields undefinable
  I always prefer null value vs undefined when possible. It forces you to input everything and create stable, unsurprising json structure. However, since elasticsearch stores the full document in the end, it's really wasted storage that quickly adds up. Also, it's probably faster to stringify like this.

• Dissociate Message and Operation
  Since there is no one structure that fits all, it makes no sense to not differentiate them. The Elasticsearch schema is still the same since it indexes both documents but we now produce different (but compatible) object.

• Removed code and title from schema
  That were never used

[linear]

NAN-2738 Set nullable fields as undefined