[Ingest Manager] Provide for conflict detection/resolution on Datasource update

[madirey]

Describe the feature:
The Datasource service does not currently provide for a way to detect conflicts when multiple updates are performed at once. This opens us up to potentially nasty race conditions, as we have several places within the app where non-atomic datasource updates are performed.

Describe a specific use case for the feature:
One potential race condition is the following scenario:

1. A user manually updates a policy from the Endpoint UI.
2. At the same time, another user adds an allowlist entry (exception).
3. In both cases, the Endpoint data sources will be retrieved, modified, and then updated... neither of the updates is atomic, so we could end up in a bad state.

Proposed solution:
Return a new version parameter when retrieving a Datasource which is equivalent to the version returned at the top level of the saved object. This version can be echoed back during Datasource update and used in the SavedObjectsClient.update() call. If multiple modifications are attempted on same version, a 409 will be returned and the client will have to try again.

Example:

//
// datasourceVersion is returned at the top level of SO response when SavedObjectsClient.get() (or create()
// or anything that returns a SavedObject) is called.
//
// It would need to be returned from DatasourceService.get() / .create() / and any of the find() functions
// and passed into DatasourceService.update() to ensure conflict resolution on updates.
//
await soClient.update<DatasourceSOAttributes>(
  SAVED_OBJECT_TYPE,
  id, {
      ...datasource,
      revision: oldDatasource.revision + 1,
      updated_at: new Date().toISOString(),
      updated_by: options?.user?.username ?? 'system',
  },
  { version: datasourceVersion }
);

[elasticmachine]

Pinging @elastic/endpoint-app-team (Feature:Endpoint)

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[jen-huang]

I'm unassigning myself for now as the current task I'm on will take some time, so opening this up for anyone else who has the bandwidth before I'm freed up again.