chore: configure dependabot for native dependency updates

[denischilik]

Background

This updates Dependabot coverage to focus on native dependency surfaces for the Flutter SDK.
It helps keep Android and iOS native dependencies current with predictable weekly update PRs.

What Has Changed

• Added Dependabot configuration for Android native dependencies in /android and /example/android.
• Added Dependabot configuration for iOS native dependencies in /ios and /example/ios.
• Added Dependabot configuration for GitHub Actions workflows.
• Scoped update limits and commit prefixes for cleaner maintenance PRs.

Screenshots/Video

N/A

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Additional Notes

This change is configuration-only and does not modify runtime SDK behavior.

[cursor]

PR Summary

Low Risk
Configuration-only; no application or runtime code changes.

Overview
Adds Dependabot via .github/dependabot.yml so dependency bumps land as scheduled PRs instead of manual updates.

GitHub Actions at repo root get weekly checks with up to 4 open PRs. Gradle for /android and /example/android use the same weekly cadence; the main Android tree allows 4 concurrent PRs and the example app 2. All entries use the dependabot label and chore commit prefix.

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