Allow setting repository custom properties

[marcoieni]

For rust-lang/infra-team#260 we need repositories custom properties. Because we'll run https://github.com/rust-lang/crabwatch on repositories that set a certain custom property (name to be decided) to true.

Tasks:

• Add the ability to set custom properties in repositories from the team repo
• Propose a name for this custom property
• Add the custom property to the crabwatch repository itself

API docs: https://docs.github.com/en/rest/repos/custom-properties?apiVersion=2026-03-10

[Sandijigs]

Here's what I'm planning for this:

• Adding the ability to set GitHub custom properties on repos from the team repo's TOML files. This follows the same approach already used for branch protection rules.
• The first property we'll use is crabwatch = true, set on the crabwatch repo. This becomes the opt-in flag that says "yes, run crabwatch on this repo."
• If individual repos ever want to customize how crabwatch behaves on them, that config will live inside the repo itself (e.g., .github/crabwatch.toml), not in the team repo. That way the repo's own maintainers can approve those changes without needing org admins. Nothing about customization in this PR though, just the opt-in flag.