Add Dependabot configuration for sia

[minder-custcodian]

This is a Minder automated pull request.

This pull request adds a Dependabot configuration to the repository to handle
package updates for multiple ecosystems.

For more information, see
https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file

[evankanderson]

I need to fix the sca_dependabot rule to append to the map if it already exists...

Possibly the best way to do this is to assemble the new file in the rule, rather than using more template magics.