Add OAuth 2.0 Token Exchange (RFC 8693) Support to PCS for PI

Add native support for the OAuth 2.0 Token Exchange (RFC 8693) grant type in Platform Common Services (PCS) for PI when integrated with external identity providers such as Microsoft Entra ID.

AIM must support exchanging externally issued access tokens for AIM-scoped tokens to enable secure, non-interactive service-to-service authentication. This capability is essential for modern enterprise architectures based on zero-trust principles, workload identity federation, and centralized identity governance.

Without token exchange support, AIM cannot fully participate in standardized OAuth 2.0 integration patterns used across enterprise environments.