Change the /login API call to use JSON instead of urlencoded POST parameters

[Crsi]

[Crsi]

It would be okay if the endpoint accepted both variants, depending on the Content-Type of the request. But dropping the variant using url-encoded arguments is a bad idea for the reason of compatibility with the OAuth workflow currently used for authentication -- this workflow requires url-encoded arguments.
However, it needs to be evaluated how the support for two different request types (depending on Content-Type) can be represented in the automatic API documentation (OpenAPI schema).