Add support for managed identity in backend authorization credentials

[Siebe Corstjens (SiebeCorstjens)]

Summary

Adds managed identity credential support to the BackendCredentialsContract DTO, enabling extraction and publishing of backend managed identity authentication configuration.

Context

The BackendCredentialsContract already supported authorization, certificate, certificateIds, header, and query credentials. Managed identity is an additional credential type supported by the Azure API Management REST API but was missing from the DTO.

Note: The official Microsoft REST API documentation does not explicitly document the managedIdentity property under BackendCredentialsContract, but the REST API does support it. This has been verified through manual testing of both extraction and publishing.

Changes

• src/common/Backend.cs: Added ManagedIdentity property to BackendCredentialsContract and introduced the BackendManagedIdentityCredential record with ClientId and Resource properties.

[Wael Kdouh (waelkdouh)]

I recommend you start using the new tool, which is the APiops CLI that we are referencing on the homepage of this repo.