Cherry-pick #22190 to 7.x: [Heartbeat] Add tls fields when connecting through proxy

[adriansr]

Cherry-pick of PR #22190 to 7.x branch. Original message:

What does this PR do?

This updates Heartbeat to enrich an event with TLS information when the connection has been established via an HTTP proxy.

Why is it important?

Because tls fields are missing from events when an HTTPS connection is performed through a proxy.

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
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Tested with tinyproxy (HTTP) and stunnel+tinyproxy (HTTPS)

diff --git a/heartbeat/heartbeat.yml b/heartbeat/heartbeat.yml
index 19c3f79e96..3ed6e835e1 100644
--- a/heartbeat/heartbeat.yml
+++ b/heartbeat/heartbeat.yml
@@ -27,7 +27,9 @@ heartbeat.monitors:
   # Human readable display name for this service in Uptime UI and elsewhere
   name: My Monitor
   # List or urls to query
-  urls: ["http://localhost:9200"]
+  urls: ["https://elastic.co/"]
+  proxy_url: "https://localhost:3333/"
+  ssl.verification_mode: none
   # Configure task schedule
   schedule: '@every 10s'
   # Total test connection and data exchange timeout

Related issues

Closes #15797

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[elasticmachine]

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Build stats

• Build Cause: Pull request #22881 opened

• Start Time: 2020-12-03T07:26:40.550+0000

• Duration: 24 min 53 sec

Test stats 🧪

Test	Results
Failed	0
Passed	932
Skipped	13
Total	945

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	932
Skipped	13
Total	945