Refactor sidecar_service method to separate port assignment

[skpratt]

Description

This separates the port assignment logic out into a helper. That way it can later be moved to a different step in the registration process.

Testing & Reproduction steps

• Refactor, no functionality changes intended.
• Unit test also refactored into separate tests.

Links

Follow-on PR

PR Checklist

• [x] updated test coverage
• [N/A] external facing docs updated
• [N/A] not a security concern