feat(cloudfront): add Distribution.addOrigin() to register origins without behaviors (#37652)

[melchii]

Adds a public addOrigin() method to Distribution that allows adding an origin to the distribution without associating it with a cache behavior. This is useful for pre-registering origins for use in origin groups or other advanced configurations.

closes #37652

Issue # (if applicable)

Closes #37652.

Reason for this change

Adding the ability to add an origin without a behavior to support dynamic routing withing cloudfront functions.

Description of changes

Added .addOrigin() which delegates to the existing internal method. It may be better to just expose the internal, but I added a new function - happy to switch to the other approach based on feedback!

Describe any new or updated permissions being added

None.

Description of how you validated changes

Added unit tests and an integration test.

Checklist

• [ X ] My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.
To suppress a specific rule, see Suppressing Rules.

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	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	48 ran	48 passed

Test	Result
No test annotations available

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
This security report is NOT a review blocker. Please try merge from main to avoid findings unrelated to the PR.
To suppress a specific rule, see Suppressing Rules.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	48 ran	48 passed

Test	Result
No test annotations available

• View Security Guardian Results with resolved templates

[laurelmay]

Thanks for your contribution and the tests! This looks ready for core team review.