Add repack support for ServerConfig.ClientAuth

[michel-laterman]

What does this PR do?

Follow up to #196 to allow fleet-server to repack the ServerConfig struct.
Extend the ClientAuthType unpack method to allow integer values and repack support. Add unit tests to verify behaviour.

Why is it important?

output from policy in fleet-server needs to be able to merge structs

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Relates Fleet Server configuration does not contain all the hosts available in the Elasticsearch cluster elastic-agent#2784

[elasticmachine]

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• Commit: 3699002