Indexing permissions as part of the Elastic Agent policy

[aleksmaus]

What does this PR do?

Implements the Indexing permissions as part of the Elastic Agent policy
#101

1. Adds a new field policy_output_permissions_hash into the Agent document that stores the hash of output_permissions from the policy.
2. Retrieves the policy from .fleet-policies upon the agent enrollment and uses the output_permissions default namespace in order to create the role descriptors for the output API key generation.
3. Checks the output_permissions upon every checking. If the hash doesn't match with the stored in the agent document the new key is generated.

This PR:

1. doesn't invalidate the old API key see the comment for the next iteration [Discuss] Indexing permissions as part of the Elastic Agent policy #101 (comment)
2. is not fully validated end-to-end with kibana for the output_permissions changes, pending on further kibana changes.

Should only be merged after the corresponding kibana changes:
elastic/kibana#94591

Why is it important?

Addresses: #101

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

Related issues

• Addresses [Discuss] Indexing permissions as part of the Elastic Agent policy #101

Screenshots

[aleksmaus]

This is the PR that was rolled back before, due to kibana changes not being merged. Now the kibana changes are merged and should be good to go.

[elasticmachine]

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Build stats

• Build Cause: Pull request #187 opened

• Start Time: 2021-03-30T15:35:53.271+0000

• Duration: 5 min 33 sec

• Commit: 98ccced

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Test	Results
Failed	0
Passed	66
Skipped	0
Total	66

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