[ML] Transforms: Edit JSON Config forgets my edits

[hendrikmuhs]

Kibana version: master:268bd095438751ec1f2e92585868927f450d68cd

Elasticsearch version: master

Describe the bug:

Steps to reproduce:

1. Create a transform using the UI, choose group_by
2. Use Edit JSON config slider
3. add something to the config, e.g. the new missing_bucket parameter

4. set some agg and finish the transform creation
5. open the job viewer, choose the just created job, check the JSON tab

Expected behavior:

the config should contain my added parameter

Actual behavior:

the config misses my edit for pivot

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[walterra]

The missing_bucket option was added in #85758. I created a meta issue to track progress about UI support for group by and aggs and their options (#86253) so will close this issue.