[Inventory] Inventory plugin

[dgieselaar]

Description

This PR adds an inventory plugin, which renders an inventory UI. Currently only data streams are rendered. This is part of the LogsAI initiative - basically we need a UI for tasks like structuring data, extracting entities, listing the results etc. This is mostly POC-level stuff. Eventually some of this code might be handed over to ECO but let's cross that bridge when we get to it.

Notes for reviewers:

@elastic/appex-ai-infra @elastic/security-generative-ai: added a truncateList utility function that takes the first n elements of an array and appends a {l-n} more string value if there are more values than n. Really simple but I expect will also be very often used because we cannot send a huge amount of items to the LLM.

@elastic/kibana-core @elastic/kibana-operations: just boiler plate stuff for adding a new plugin (and thank you for enabling us to run quick_checks locally!

@elastic/obs-knowledge-team: added support for streaming using an Observable.

@elastic/obs-ux-management-team: added links to the Inventory UI in the Observability plugin

@elastic/obs-entities: I've added an entity manager client to be able to fetch entity definitions on the server. Maybe there's a better way? LMK.

@elastic/obs-ux-logs-team: added a deeplink to the Inventory UI. I've also moved CODEOWNERS for this package to @elastic/obs-ux-management-team as they own the Observability plugin where this is mostly used.