[Design] Support External Evaluation Policy (Invoke) in management SDKs #50818

Summary

Design support for Azure Policy External Evaluation ("Invoke") in Java management SDKs.

The SDK should handle the following flow:

1. Detect 403 RequestDisallowedByPolicy with missingPolicyTokenDetails.
2. Call acquirePolicyToken using the original request URI, method, and payload.
3. Add the returned token to x-ms-policy-external-evaluations.
4. Replay the original operation once.

Cross-language context and service clarifications are tracked in:
Azure/azure-sdk#10081

An initial Java prototype is available at:
#49701

Byte-for-byte payload requirement

For an operation with request-body byte sequence B, the following must use the same serialized bytes:

1. The original ARM operation body.
2. acquirePolicyToken.operation.content.
3. The replayed ARM operation body.

Logical JSON equality is insufficient. Deserializing and reserializing the payload may change property ordering, whitespace, escaping, number formatting, null handling, or encoding and invalidate the token's content binding.

The implementation should retain the final serialized request content and reuse it without modification for token acquisition and replay. Bodyless operations such as DELETE should preserve the service-defined empty/null content behavior.

Design goals

• Work across management libraries without service-specific changes.
• Support PUT, PATCH, POST, and DELETE operations.
• Preserve the exact serialized request content without unnecessary overhead.
• Prevent recursive acquire/replay loops.
• Surface policy-evaluation and token-acquisition failures clearly.
• Define tracing, logging, and token-redaction behavior.
• Decide whether support is default-enabled or explicitly configured.

Expected outcome

Please propose the Java-specific architecture and public API, preferably with a prototype covering:

• successful 403 → acquirePolicyToken → replay;
• verification that the original, token-acquisition, and replay payloads are byte-for-byte identical;
• prevention of payload reserialization;
• bodyless operations;
• token-acquisition failure or policy denial;
• replay-loop prevention;
• synchronous and asynchronous APIs;
• integration with the management client pipeline.

The prototype is for design validation and does not need to be production-ready.