Add userAssignedIdentityId support for command jobs

[saanikaguptamicrosoft]

1. MI has been moved out of env vars. So it was leading to this error:

Error Code: ScriptExecution.StreamAccess.Unexpected
Native Error: error in streaming from input data sources
        StreamError(Unknown("An unexpected error occurred while resolving AccessToken: CredentialUnavailableError: ManagedIdentityCredential authentication unavailable. The requested identity has not been assigned to this resource. Error: Unexpected response \"{'error': 'invalid_request', 'error_description': 'Identity not found'}\"", None))
=> An unexpected error occurred while resolving AccessToken: CredentialUnavailableError: ManagedIdentityCredential authentication unavailable. The requested identity has not been assigned to this resource. Error: Unexpected response "{'error': 'invalid_request', 'error_description': 'Identity not found'}"
        Unknown("An unexpected error occurred while resolving AccessToken: CredentialUnavailableError: ManagedIdentityCredential authentication unavailable. The requested identity has not been assigned to this resource. Error: Unexpected response \"{'er

2. Now job submission succeeds: https://eastus2euap.ai.azure.com/nextgen/r/csA7805pQa-VMt_Nw-7-9A,shared-finetuning-rg,,fdp-command-job-CANARY,fdp-command-job-UMI-CANARY-proj/build/train/custom-jobs/olden_plastic_y4lv5tn3rs/details?flight=horizon_train%2Chorizon_compute%2Chorizon_intcdn