[Ingest Pipelines] Improvements to the "Foreach" processor form

[jloleysens]

Describe the feature:

The "Foreach" processor currently provides users with a JSON editor in which to provide a processor definition that should be executed against each value in a document:

This creates a situation where users can provide bad configuration which can lead to a bad UX. This form should be improved to prevent users from providing bad input. We should:

• give users the ability to choose from a list of known processors
• present them with the associated form for that processor where they can enter values
• take that result and store it in the "Foreach" processor

This could be solved by embedding another form inside of the form in the flyout that will function as a "field" from the perspective of the encompassing form.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[elasticmachine]

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