Native ads request

[bparrishMines]

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

List which issues are fixed by this PR. You must list at least one issue.

Pre-Review Checklist

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• I linked to at least one issue that this PR fixes in the description above.
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• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request effectively expands the AdsRequest API for both Android and iOS, providing more control over ad requests. The changes are well-implemented across Dart, Kotlin, and Swift, with corresponding updates to Pigeon definitions and comprehensive tests. The upgrade to a newer Pigeon version also introduces beneficial changes for testability. I have one minor suggestion to improve the robustness of the Swift implementation by replacing a force cast with a safer conditional cast. I've referenced the Flutter style guide in my review comment.

[gemini-code-assist]

Using a force cast as! can lead to a runtime crash if the type cast fails. For better robustness and maintainability, it's recommended to use a conditional cast as? with a guard statement to safely unwrap the delegate.

        guard let proxyApiDelegate = pigeonApi.pigeonRegistrar.apiDelegate as? ProxyApiDelegate else {
          throw PigeonError(
            code: "cast-error",
            message: "apiDelegate is not of type ProxyApiDelegate.",
            details: nil)
        }
        throw proxyApiDelegate.createConstructorNullError(
          type: URL.Type.self, parameters: ["string": contentURL])