Refactor fleet policy APIs, fix JSON issues

[fearful-symmetry]

Fixes #127

What does this PR do?

This PR does a few things:

• Update the JSON objects being sent to the fleet API to fix a few bugs as documented in CreatePolicy() sends incorrect JSON requests #127. Most of these bugs were due to small things like missing omitempty fields.
• Change the structs used by the go APIs to reflect the fields actually accepted/returned by Kibana, in order to make the API usage clearer.
• Reference the openAPI docs in Kibana where needed
• Add some integration tests. These require a Kibana instance, but I don't think we have any built-in e2e tests ready-to-use in this repo.
• Add a DeletePolicy() helper
• Fix some of the error handling in client.go

Why is it important?

This is blocking integration test development.

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

[elasticmachine]

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• Start Time: 2023-06-06T21:06:00.597+0000

• Duration: 10 min 0 sec

Test stats 🧪

Test	Results
Failed	0
Passed	787
Skipped	11
Total	798

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[ycombinator]

LGTM. Thanks for making these fixes and improvements.