Add helper to provide default values to rego.Runtime()

[anderseknert]

The way the runtime term is set up in the server and elsewhere makes use of internal packages, making it harder than it has to be for users of the Rego API to populate runtime values similar to what users of the OPA server or SDK get out of the box. It would be great if we offered a public function like DefaultRuntime() to provide this.

[stale[bot]]

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