🌊 Streams: Handle stale classic streams

[flash1293]

An invariant we have to handle somehow is if a user made additions to an unwired data stream via the streams interface, then the underlying data stream gets deleted.

This is "allowed", since the data stream is not managed by streams.

Currently, the UI breaks if this happens and shows error toasts when trying to load the doc count or when trying to change processing.

This PR makes this a regular case the API can handle:

• The GET /api/streams/<id> endpoint does not throw, but still returns the existing definition. A new key data_stream_exists indicates whether we are in this situation
• The UI clearly communicates to the user and doesn't try to do additional requests

Trying to update ingest via the API will still result in an error.

Another weird behavior related to that was that if a dashboard is linked to a stale classic stream, no definition is ever saved and the stream disappears from the list when deleted, making the dashboard link inaccessible. This PR introduces ensureStream which is called by the dashboard APIs and makes sure the definition is there if dashboard links exist. As a side effect, this makes sure that a user can't add dashboard links to a stream they don't have access to - IMHO we should have done that from the start.

This does not touch wired streams - for those, the data stream getting deleted is a breach of contract. We should still handle it gracefully, but in this case I think we should go another route and offer a button in the UI to use the "resync" API to reconcile the state of the streams layer and Elasticsearch. I will look into this on a separate PR.

[elasticmachine]

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Metrics [docs]

Public APIs missing comments

Total count of every public API that lacks a comment. Target amount is 0. Run node scripts/build_api_docs --plugin [yourplugin] --stats comments for more detailed information.

id	before	after	diff
@kbn/streams-schema	201	206	+5

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
streamsApp	280.1KB	281.1KB	+1012.0B

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API count

id	before	after	diff
@kbn/streams-schema	201	206	+5