[ES UI Shared] [Form hook] Fields with same path get cleaned up on re-mounts of form

[jloleysens]

Description

Checkout 1a52d64 (see this PR for context #72849).

When adding a new processor using one of the new forms inside of the Ingest Processor Editor and switching between processor types, for example "append" -> "bytes" in the flyout (click "Add Processor" at the bottom of the list of processors). Form fields with the same name for "append" and "bytes" are incorrectly cleaned up on the form object in memory.

A side effect of this is that when going from "append" to "bytes", both have a field on path "fields.field" which is removed and so does not have validation applied on form submission

Steps to reproduce

1. Go to Ingest Pipelines in stack management
2. Add a new pipeline
3. Select "append" processor type
4. Click "Create" at the bottom of the flyout to trigger validations
5. Select "bytes" processor type
6. Click "Create" at the bottom of the flyout to trigger validations

The expected behaviour after step 6 is that the form validation should block you from creating a "bytes" processor without providing a value for field, but on the provided commit it does not.

[elasticmachine]

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