Add support for repacking and merging tls.Config structs

[michel-laterman]

What does this PR do?

Add support for the Unpack operation for tls.Config attributes that are specified as strings but stored as other values. The existing Unpack operations are limited to string only and these will cause issues when trying to merge previously unpacked structs.

Why is it important?

We are tying to get fleet-server instances running in agent mode to reload its output block using values retrieved from the policy. See the pr
Merging the changed output value into the existing config is needed to support this.
However we are running into an error string verification mode must be an identifier accessing 'inputs.0.server.ssl.verification_mode' accessing 'inputs.0.server.ssl' that occurs because we are working with structs that have been already unpacked.
The types that cause issue are those that are serialized as a string but represented in the struct as a numeric value.

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]

💚 Build Succeeded

• Buildkite Build
• Commit: 93dc19f

[jlind23]

@pierrehilbert this is pending on the Elastic Agent team, can we get some traction please?