Add support for OPA management APIs

[philipaconrad]

Quoting @tsandall from open-policy-agent/swift-opa#57:

---

swift-opa lacks support for the OPA management APIs which means that users currently have to DIY integration between swift-opa and their control plane. To reach parity with the Go-based implementation, we should extend swift-opa to minimally support library-based use cases where users construct an instance of the OPA engine based on an OPA configuration file. The OPA engine should support discovery, bundles, and decision logs. The Go-based implementation includes an SDK package that essentially exposes two functions (pseudocode below). We could model the swift SDK based on that.

Links:

• Go SDK: https://pkg.go.dev/github.com/open-policy-agent/opa/v1/sdk
• Management APIs: https://www.openpolicyagent.org/docs/management-introduction
• OPA configuration: https://www.openpolicyagent.org/docs/configuration

Proposed API (pseudocode):

New(Config) => OPA

OPA.Evaluate(Input) => Result

struct Input {
    Path string
    Value JSON
     ... other fields such as current time
}


struct Result {
   Value JSON
    ... other fields such as metrics, debug output, etc.
}

For the MVP we can exclude support for:

• Status API support
• Direct read/write access to the in-memory store
• Signing
• Distributed tracing
• Server settings (not supported)
• Disk storage

To implement support for bundles, we will need #28