[Fleet] Add 'Used by # agent policies" to the Integration Details header

[paul-tavares]

Part of: Integration Specific Policy Views

Description

On the "Integrations > {package} > Detail" page, show the total number of (unique) Agent policies where the given Integration is being used. This component should only be shown if the package in installed

Mock:

[elasticmachine]

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

[paul-tavares]

@bfishel (@hbharding ),
I forget what we decided on for the header component text as far as the work "policies". Should it be "used by # policies"? or "Used by # Agent policies"?

[hbharding]

We should use "Agent Policies" to avoid ambiguity. I might rearrange the text so that it's a little more consistent with how we show counts in our tables. I prefer the right option in the screenshot below.