[Ingest Node Pipelines] New description field for processors

[jloleysens]

Describe the feature:

Building UI for the managing processors in Kibana has shown how the tag field may be used to serve dual purposes:

1. A unique identifier for a processor instance
2. A descriptive name for the processor instance

It would be really useful to have an extra (optional) description field when creating a processor so that the tag can be more singular in its concern.

CC @jakelandis @talevy

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Ingest)

[jethr0null]

To add a bit of context around how we'd like to use this field, I'll provide an example in the context of the Ingest Nodes Pipeline UI that the team is currently working on. When displaying a list of processors within a pipeline visually, it's more difficult to determine which processor is responsible for which changes without the presence of a processor description. Here is an example mockup that highlights where these descriptions would be used in order to make it easier for users to quickly see information about the processor beyond the processor name and identify the specific processor they intend to edit/delete/etc.