Preview an external-source import

{  "source": "string",  "sourceKind": "github",  "upstreamHash": "string",  "candidates": [    {      "name": "string",      "description": "string",      "path": "string",      "manifestValid": true,      "warnings": [        "string"      ]    }  ],  "stars": null}

{  "defined": true,  "code": "UNAUTHORIZED",  "status": 401,  "message": "Authentication required or credentials invalid.",  "data": null}

{  "defined": true,  "code": "NOT_FOUND",  "status": 404,  "message": "The requested resource does not exist.",  "data": {    "resource": "string"  }}

{  "defined": true,  "code": "VALIDATION",  "status": 422,  "message": "The input failed domain validation.",  "data": {    "issues": [      {        "path": "string",        "message": "string"      }    ]  }}

{  "defined": true,  "code": "RATE_LIMITED",  "status": 429,  "message": "Rate limit exceeded; retry later.",  "data": {    "retryAfterSeconds": -9007199254740991  }}