[webview_flutter]: Support WKNavigationType

[tranductam2802]

Support return NavigationType in NavigationRequest from WKWebview.

Detail:

• In iOS, have no way to determine onNavigationRequest was fired by users clicks on a link or programmatically re-direct.
• Integrate WKNavigationType into webview_flutter

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or this PR is [exempt from CHANGELOG changes].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[stuartmorgan]

Thanks for the contribution!

Please see https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#platform-support for context on how we develop our plugins; directly embedding iOS-specific constants in the platform interface is not how we would implement navigation type detection.

Please see https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changing-federated-plugins as well, and also the missing items in the PR description checklist, for things that this PR would need in order to proceed.

Marking as a draft for now, as this is not yet in a reviewable state.