feat!: expose turn-by-turn event payloads

[illuminati1911]

• Add typed turn-by-turn event and step payloads.
• Align Android and iOS payload fields, including roundabout exit numbers.
• Add integration coverage and usage documentation.

BREAKING CHANGE: TurnByTurnEvent now declares its native payload fields. Code that manually constructs these events must provide the required fields.

Fixes: #641

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation
• I added new tests to check the change I am making
• All existing and new tests are passing.