Incorrect parameter type on role assignment create operation

[This parameter](https://github.com/Azure/azure-rest-api-specs/blob/f4729014caa04727c86f37bad1d1b36305942297/specification/authorization/resource-manager/Microsoft.Authorization/2015-07-01/authorization.json#L417) should be modeled as a UUID, not a string.
Service returns error:
`"The role assignment ID 'Whatever' is not valid. The role assignment ID must be a GUID."`

Open specification/authorization/resource-manager/Microsoft.Authorization/2015-07-01/authorization.json at the parameter near line 417. Confirm how that role assignment create parameter is modeled and change its type from string to UUID, then validate that the specification represents the service's GUID requirement.

Written by the indexing model from the issue text.