Add Cargo dependabot support

[MikeMcQuaid]

• add cargo directories to the shared template
• keep cargo updates when Cargo.toml is present

May be needed for Homebrew/brew#21774

[Copilot]

Pull request overview

Adds Cargo (Rust) Dependabot support to the shared Dependabot template and ensures the sync script keeps Cargo updates only when a Cargo.toml is present, aligning the org-wide config generation with repos that include Rust code (e.g., Homebrew/brew).

Changes:

• Add Cargo ecosystem detection (Cargo.toml) to the shared sync script’s Dependabot filtering logic.
• Add a Cargo Dependabot entry (with multi-directory support) to the shared Dependabot template.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/actions/sync/shared-config.rb	Recognizes cargo updates and filters them based on presence of Cargo.toml in configured directories.
.github/actions/sync/dependabot.template.yml	Introduces a Cargo update block covering / and /Library/Homebrew/rust/brew-rs.

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