Reindex wizard "morph mappings" feature

[cjcenizal]

Blocked by #46755

Users often reindex to fix mistakes with mappings by:

• Adding new fields. Involves specifying a new field and optionally adding a transform to populate the field with data.
• Removing unwanted fields. Involves explicitly leaving out an existing field and adding a transform to prevent the field from being populated.
• Renaming a field. Involves specifying a new field and adding a transform to populate the new field with data while preventing the old field from being populated.
• Merge multiple fields into a single field. Involves specifying a new field and adding a transform to populate the field with data while preventing the old fields from being populated.

A "morph mappings" feature would support these use cases by allowing the user to visually add/remove fields and translate existing fields to new ones. Under the hood, it would intelligently reconcile multiple references to the same fields and build the right mappings configuration and pipeline or painless script.

As a side benefit, teams could use this feature to build migrations for use with the Upgrade Assistant.

[elasticmachine]

Pinging @elastic/es-ui

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)