Add more Fleet APIs to Kibana client

[ycombinator]

What does this PR do?

This PR enhances the Kibana client to implement methods for the following APIs:

• List Fleet Server hosts
• Get Fleet Server host
• Get agent policy

Why is it important?

These APIs will be useful for integration tests to determine the Fleet Server URL for a Fleet-managed policy they're testing with. See https:/elastic/elastic-agent/pull/2618/files#r1188775382.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Author's Checklist

• [ ]

Related issues

• Follow up to Add Fleet APIs to Kibana client #118

[elasticmachine]

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• Start Time: 2023-05-09T19:12:30.817+0000

• Duration: 8 min 16 sec

Test stats 🧪

Test	Results
Failed	0
Passed	783
Skipped	5
Total	788

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[blakerouse]

Looks good.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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