[Fleet] change UX of edit integration policy agent policies

[juliaElastic]

Summary

Closes https://github.com/elastic/kibana/issues/186629

Changed UX of edit integration policy page to update agent policies.

To verify:

• Add integration or create new agent policy with system integration
• Edit the integration policy
• Verify that the UI allows adding new agent policy with making changes to existing agent policies

Checklist

Delete any items that are not applicable to this PR.

• [x] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [x] Unit or functional tests were updated or added to match the most common scenarios