All ML objects stored in internal indices are currently parsed
strictly. This means unknown fields lead to parsing failures.
In turn, this means we cannot add new fields in any of those
objects (e.g. bucket, record, calendar, etc.) as it is not
backwards compatible.
This commit changes this by introducing lenient parsing when
it comes to reading those objects from the internal indices.
Note we still use strict parsing for the objects we read from
the c++ process, which is nice as it guarantees we would detect
if any of the fields were renamed on one side but not the other.
Also note that even though this is going in from 6.3, we cannot
introduce new fields until 7.0.
relates elastic/x-pack-elasticsearch#4232
Original commit: elastic/x-pack-elasticsearch@3f95d3c7b9
2aeff7713c