[deep link][ios] load links for ios and change path to nullable

[hangyujin]

1. load ios links,
2. change path to nullable because ios can only get domains from the app side, the paths are on the web side.

issue: https://github.com/flutter/devtools/issues/7799

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or there is a reason for not adding tests.

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