certutil: Add functional options

[AndersonQ]

What does this PR do?

Add functional options to NewRootCA and GenerateChildCert to allow setting a prefix for the CN and multiple DNS names.

Also, it fixes GenerateChildCert generating a RSA instead of EC certificate

Why is it important?

• set CN prefix: when generating and using multiple certificates, such as to configure mTLS, it's hard to debug any issue if all certificates have the same CN
• add multiple DNS names: when trying to simulate a real situation, it might require a certificate to have multiple DNSs

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

Author's Checklist

• Relates Improve test proxy/mock fleet server to proxy HTTPS connections elastic-agent#4903

[AndersonQ]

closed in favour of #240

[elasticmachine]

💚 Build Succeeded

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• Commit: 1656938

cc @AndersonQ