FR: allow changing discriminators in an existing resource (via additionalProperties in MergePatch)

[blakew]

Because different discriminator values can have different schemas, our current strategy of applying basic JSON merge-patch fails when users change the discriminator and (therefore the schema).

Ex:

ex. target:
{
  ...
  limit: {
    method: 'row_count',
    threshold: 1000
  }
  ...
} 
PATCH:
{
  ...
  limit: {
    method: 'percentile',
    percentile: 80
  }
  ...
} 

AJV complains that percentile isn't valid because it's seeing both threshold and percentile when applying naive json merge-patch.

Per @emk - The basic fix here is for all MergePatch types to start allowing additionalProperties: { type: null }.

Then this would work, for ex:

PATCH ...

{
  ...
  limit: {
    method: 'percentile',
    percentile: 80,
    threshold: null
  }
  ...
}