Add data_streams.wait_for_integration

[axw]

Motivation/summary

Add apm-server.data_streams.wait_for_integration config. When both this and apm-server.data_streams.enabled are true, the server will wait at startup until the APM integration package is installed.

For now this requires apm-server.kibana to be configured, and will query the Fleet API to query the integration package status. We should follow up with a parallel query to Elasticsearch (if configured) to check if the index templates have been created.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

How to test these changes

1. Create a fresh cluster with no integration isntall
2. Run apm-server with apm-server.data_streams.enable=true (wait_for_integration defaults to true)
3. Observe that apm-server logs that it is waiting for the integration, and it does not start listening on 8200
4. Install the apm integration package in Kibana
5. Observe that apm-server continues, and starts listening on 8200
6. Send some events
7. Check that they are indexed into data streams

Related issues

#5914

[apmmachine]

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Build stats

• Start Time: 2021-08-11T12:53:38.237+0000

• Duration: 44 min 47 sec

• Commit: d7bdf49

Test stats 🧪

Test	Results
Failed	0
Passed	5918
Skipped	14
Total	5932

Trends 🧪

[simitt]

Haven't run it but changes look good.

[axw]

/test

[stuartnelson3]

Confirmed with BC3