chore: add Dependabot dependency update automation

[j-kon]

Summary

Add Dependabot configuration so bdk-dart can automatically surface dependency update PRs for the package, demo app, Rust native crate, and GitHub Actions.

This should help maintainers notice outdated dependencies earlier without manually checking each ecosystem.

Scope

• Add .github/dependabot.yml.
• Configure updates for root Dart/pub dependencies.
• Configure updates for bdk_demo Dart/Flutter dependencies.
• Configure updates for Rust/Cargo dependencies under native.
• Configure updates for GitHub Actions used by repository workflows.
• Use a low-noise schedule, such as weekly updates.
• Group related updates where it keeps review manageable.

Out of scope

• Updating dependency versions in this PR.
• Changing lockfiles.
• Changing CI workflow behavior.
• Changing native build or release logic.
• Enabling auto-merge.

Acceptance criteria

• Dependabot is configured for pub, Cargo, and GitHub Actions ecosystems.
• The configuration covers the repository root, bdk_demo, native, and .github/workflows.
• The update schedule is intentionally low noise.
• The PR only adds Dependabot configuration unless maintainers request otherwise.
• The configuration is valid YAML and follows GitHub Dependabot conventions.