[Fleet] Add space ID to Fleet system indices

[kpollich]

#181860 details the process of moving Fleet's saved objects to a new single-space affinity model. In addition to this work, we need to make Fleet's non-saved-object data segregated by space.

Relevant dev docs:

kibana/x-pack/plugins/fleet/dev_docs/data_model.md

Lines 22 to 82 in 01af646

	## `.fleet-*` Indices
	For any data that needs to be accessible by Fleet Service instances to push updates to, we write and read data
	directly to a handful of `.fleet-` Elasticsearch indices. Fleet Server instances are configured with an API key that
	has access only to these indices.
	In prior alpha versions of Fleet, this data was also stored in Saved Objects because Elastic Agent instances were
	communicating directly with Kibana for policy updates. Once Fleet Server was introduced, that data was migrated to these
	Elasticsearch indices to be readable by Fleet Server.
	_Note: All of these system indices are plain indices, and not data streams._
	### `.fleet-agents` index
	Each document in this index tracks an individual Elastic Agent's enrollment in the Fleet, which policy it is current
	assigned to, its check in status, which packages are currently installed, and other metadata about the Agent.
	All of the code that interacts with this index is currently located in
	[`x-pack/plugins/fleet/server/services/agents/crud.ts`](../server/services/agents/crud.ts) and the schema of these
	documents is maintained by the `FleetServerAgent` TypeScript interface.
	- Cleanup model: N/A
	### `.fleet-actions` index
	Each document in this index represents an action that was initiated by a user and needs to be processed by Fleet Server
	and sent to any agents that it applies to. Actions can apply to one or more agents. There are different types of actions
	that can be created such as policy changes, unenrollments, upgrades, etc. See the `AgentActionType` type for a complete
	list.
	The total schema for actions is represented by the `FleetServerAgentAction` type.
	- Cleanup model: Fleet Server considers actions expired after 30 days, and will remove them via an hourly process
	- [Source](https://github.com/elastic/fleet-server/blob/9af3b2176b42a0de34c5583b5430558c03792dd0/internal/pkg/gc/schedules.go#L29-L33)
	### `.fleet-actions-results`
	- Cleanup model: N/A
	### `.fleet-servers`
	- Cleanup model: N/A
	### `.fleet-artifacts`
	- Cleanup model: N/A
	### `.fleet-enrollment-api-keys`
	- Cleanup model: Deleteable via Fleet UI/API, deleted when an agent policy is deleted
	- [Source](https://github.com/elastic/kibana/blob/7a35748cb43f2c73623ffda6fa02b91c3cb4c689/x-pack/plugins/fleet/server/services/api_keys/enrollment_api_key.ts#L102)
	### `.fleet-policies`
	- Cleanup model: Deleted when a corresponding agent policy is deleted in the Fleet UI or API
	- [Source](https://github.com/elastic/kibana/blob/976b1b2331371f4a1325f6947d38d1f4de7a7254/x-pack/plugins/fleet/server/services/agent_policy.ts#L699-L701)
	### `.fleet-policies-leader`
	- Cleanup model: N/A

Implementation

• Add space ID to .fleet-enrollment-api-keys documents
• Add space ID to .fleet-fileds-* indices - Link
• Add space ID to .fleet-actions and .fleet-actions-results
• Add space ID to .fleet-policies

It's unclear whether the following system indices are still in use or not, and whether or not they should be updated

• .fleet-servers
• .fleet-artifacts
• .fleet-policies-leader

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

@kpollich I am about to start that one as it will unblock a bunch of issues, what do you think of using the namespaces: keyword mapping here?, consistent with SO, (and it could potentially allow us to support multiple namespace in a long future)

[kpollich]

namespaces: keyword for the mapping type w/ an array of keywords for the space ID makes sense to me 👍