[Fleet] Ensure integrations can be installed across many spaces within new space awareness model

[kpollich]

Blocked by #181860

Once Fleet's new space awareness feature is implemented, we need to test whether integrations can be installed in multiple spaces. It's a very common request for us to support sharing integration assets between many spaces. In theory, with Fleet's Saved Object's being designated as namespaceType: single, we should be able to simply install the integration in each space, and each Installation saved object will be tracked separately with its own asset references.

We'll want to audit the behavior of integrations in this situation, as #176552 will likely result in many "hardcoded" links in Markdown panels not working. This behavior should be documented and captured in a follow-up as appropriate.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

This may supersede #172963 depending on the results of the testing.

[kpollich]

After some discussion with @nchaulet I'm closing this. We're going to use the approach proposed in #172963 to keep the epm-packages SO type agnostic and use a "custom" space awareness model built on top of the existing kibana_space_id property on that SO type. The complexities around managing space-aware integration installation alongside non-space-aware ES resources like index templates and ingest pipelines means we need to rely on our own business logic rather than Kibana's core space APIs in this case.