[8.14](backport #4739) [CI] Separated extended testing pipeline

[mergify]

What does this PR do?

Depends on #4738
Extracting long running integration tests to a separate pipeline.
Added changeset filters to the main the new pipeline
Added notification context for integration tests

Important: Introduces a concurrency group for each type of exteneded tests. The concurrency groups are limited to 8. That means we can run only 8 jobs of a type simultaneously

For example:

steps:
  - label: "Serverless integration test"
    key: "serverless-integration-tests"
    concurrency_group: elastic-agent-extended-testing/serverless-integration # <- concurrency group
    concurrency: 8 # <- concurrency limit
    env:
      BEAT_NAME: "elastic-agent"
    command: ...

Why is it important?

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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

• Relates Skip integration tests for changes that do not modify Go source files #3674
• Relates Separate expensive test in elastic-agent CI to a separate pipeline #4710

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

---

This is an automatic backport of pull request #4739 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pazone? 🙏