feat: add traffic delay severity

[Samo8]

Description

Adds TrafficDelaySeverity enum (light, medium, heavy, noData) to NavigationTimeAndDistance and RemainingTimeOrDistanceChangedEvent, enabling apps to display real-time traffic delay information during navigation.

Implemented for both Android and iOS platforms with appropriate platform-specific conversions.

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 (added documentation comments to new types and fields)
• I added new tests to check the change I am making
• All existing and new tests are passing.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[jokerttu]

Thank you for this PR.
We'll take a look of the implementation.

Do not worry about intergration test fails as those are blocked for third-party developers for security reasons.
We'll run the tests trough locally on review process.

[illuminati1911]

@Samo8 Please also add this functionality to the navigation page on our example app.
example/lib/pages/navigation.dart

There should already be a listener and minimalist UI for showing remaining distance and time so just add this there.

[Samo8]

@illuminati1911 updated

[illuminati1911]

Thank you for your hard work. Looks good to me!