[Heartbeat] redact authorization headers from logger

[vigneshshanmugam]

• Heartbeat allows users to pass custom headers for monitors when doing Lightweight HTTP checks. Any errors thrown on the configuration failure would result in configuration printed to the stdout including the sensitive headers (auth) in this case.
• This PR redacts the authorization headers by adding them to the logger and also makes sure we capture both lowercase and uppercase strings as both are allowed for HTTP header checks. Added tests for the same

Heartbeat logs after this PR

ERROR [autodiscover]	autodiscover/autodiscover.go:212	Auto discover config check failed for config '{
  "headers": {
    "Authorization": "xxxxx",
    "Content-Type": "application/json"
  },
  "hosts": [
    "xxxxx"
  ],
  "id": "test",
  "schedule": "@every 10s",
  "type": "tcp"
}', won't start runner: monitor ID test is configured for multiple monitors! IDs must be unique values.

[elasticmachine]

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• Start Time: 2021-07-15T14:43:24.450+0000

• Duration: 144 min 0 sec

• Commit: b943cce

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Failed	0
Passed	49267
Skipped	5396
Total	54663

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Passed	49267
Skipped	5396
Total	54663

[vigneshshanmugam]

/test

[urso]

Let's add proxy-authorization as well. We have a few inputs (including heartbeat) that allow users to configure proxies.

[vigneshshanmugam]

Done 👍🏽

[urso]

Thank you! Please backport to 7.x, 7.14, and 7.13 branches.