Cherry-pick #12867 to 7.x: Add "mechanism" in output.kafka to support SCRAM-SHA-512 and SCRAM-SHA-256

[jsoriano]

Cherry-pick of PR #12867, #23103 and #22342 to 7.x branch. Original message:

New config key mechanism was introduced.
How to use it:

output.kafka:
  codec.format:
    string: '%{[@timestamp]} %{[message]}'

  hosts: ["localhost:9092"]
  topic: 'mytopic'
  partition.round_robin:
    reachable_only: false
  required_acks: 1
  compression: gzip
  max_message_bytes: 1000000
  username: user
  password: pass
  sasl.mechanism: SCRAM-SHA-512

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #23110 updated

• Start Time: 2020-12-14T20:30:20.584+0000

• Duration: 137 min 38 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17410
Skipped	1404
Total	18814

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Terraform Apply on x-pack/metricbeat/module/aws

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Terraform Apply on x-pack/metricbeat/module/aws

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Test stats 🧪

Test	Results
Failed	0
Passed	17410
Skipped	1404
Total	18814

[jsoriano]

Packaging failure with heartbeat will be fixed by #23111.

[jsoriano]

Packetbeat packaging failed too, but I cannot reproduce this, launching packaging again...