feat(destinations): add new fields to destinations; secureUrl, custom headers auth and more

[nzur-newrelic]

Description

This PR adds logic in regards to 2 new fields on destinations. Custom Headers Auth- Another type of auth, which uses a list of key values. SecureUrl- A new and secure way to add url to your destination, now separated into a a non-secure prefix and a secure suffix. Please include a summary of the change and which issue is fixed (if relevant).

Requires https://github.com/newrelic/newrelic-client-go/pull/1122

Type of change

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• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

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• [x] My commit message follows conventional commits
• [x] My code is formatted to Go standards
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes. Go here for instructions on running tests locally.

How to test this change?

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