WIP: decrypt client TLS certificate key for Elastic Defend

[AndersonQ]

It adds EndpointTLSComponentModifier which will check if the client certificate key is encrypted, if so, it'll decrypt the key and pass it decrypted to endpoint (Elastic Defend)

What does this PR do?

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

Author's checklist

Acceptance Criteria:

• The Elastic Agent injects the decrypted private key into the fleet.ssl.key configuration setting within Elastic Defend's configuration.
• Unit tests verify that the decrypted key is correctly injected into the Elastic Defend configuration.
  • extend the TestEndpointSignedComponentModifier to include a check for the mTLS client certificate key decryption.
  • Integration tests: Out of scope for this ticket as it's require Improve test proxy/mock fleet server to support further elastic-agent TLS tests #4903 to be done first.

Disruptive User Impact

How to test this PR locally

Related issues

• Closes Pass the decrypted mTLS client certificate key to Elastic Defend #5490

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?
• ...

[mergify]

This pull request does not have a backport label. Could you fix it @AndersonQ? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-v8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[elastic-sonarqube]

Quality Gate passed

Issues
1 New issue
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
89.1% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube