chore(ci): add dependabot config for python deps and github actions

[maxlambrecht]

What

Add a Dependabot configuration to manage dependencies for both Python packages (spiffe, spiffe-tls) and GitHub Actions.

• Weekly update cadence
• Conventional Commit prefixes for PR titles
• Logical grouping of runtime vs dev dependencies
• Grouped GitHub Actions updates

Why

• Keep runtime and development dependencies reasonably fresh without creating PR noise
• Ensure consistent Conventional Commit titles for automation and changelog tooling
• Reduce maintenance overhead and security exposure from stale dependencies
• Standardize dependency management across both library packages and CI