Add NewWithConfig init function to api/server.go

[fearful-symmetry]

What does this PR do?

This adds a new init method, NewWithConfig, that takes a Config struct, and not a config.C blob.

Why is it important?

As part of elastic/elastic-agent#4499 I'm refactoring the server setup process so the monitoring server can properly be reloaded by agent. I need this to clean up the init process, since agent will needlessly turn the Config struct into a config blob so it can pass it to New.

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

[elasticmachine]

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cc @fearful-symmetry