fix: deprecate requirement of session token for service accounts (#9320)

This PR fixes couple of behaviors with service accounts

- not need to have session token for service accounts
- service accounts can be generated by any user for themselves
  implicitly, with a valid signature.
- policy input for AddNewServiceAccount API is not fully typed
  allowing for validation before it is sent to the server.
- also bring in additional context for admin API errors if any
  when replying back to client.
- deprecate GetServiceAccount API as we do not need to reply
  back session tokens

cmd/admin-handlers.go

@@ -942,7 +942,7 @@ func toAdminAPIErr(ctx context.Context, err error) APIError {
 				HTTPStatusCode: http.StatusNotFound,
 			}
 		} else {
-			apiErr = errorCodes.ToAPIErr(toAdminAPIErrCode(ctx, err))
+			apiErr = errorCodes.ToAPIErrWithErr(toAdminAPIErrCode(ctx, err), err)
 		}
 	}
 	return apiErr