[Bug] Support library-agnostic HTTP clients for Service Fabric managed identity #965

Context

The certificate-pinning change in #952 tied Service Fabric authentication to requests.Session, affecting Azure Identity’s Azure Core integration during MSAL 1.38 uptake for [Azure/azure-sdk-for-python#48708](Azure/azure-sdk-for-python#48708).

PR #964 fixes adapter-subclass rejection, but the broader Requests-specific API restriction remains.

Request

Provide a library-neutral transport capability for certificate pinning so Azure Identity can retain its pipeline without importing Requests, and other HTTP libraries can integrate.

Keep Requests as the default backend, preserve certificate validation before sending the Secret header, and explicitly reject transports that cannot enforce it. Verify the contract with Requests and a non-Requests backend, including Azure Identity’s public Service Fabric credential APIs.