[client] Refactor operation method signature

[berislavlopac]

Is your feature request related to a problem? Please describe.
Currently, the operation methods attached to the client expect *args for path parameters and **kwargs for query parameters, while there are keyword arguments body_ and headers_ for the respective types of operation parameters. One problem here is that there is no way to update other parameter types (specifically cookies), but also the number of "special" kwargs seems a bit excessive.

Describe the solution you'd like
Instead of separate kwargs for different types we could define a single argument, named extra_, which would accept an arbitrary dictionary. Internally, the method would extract any values that match special keys (namely headers, cookies and body, but also possibly files and others). The *args and **kwargs would remain reserved for path and query parameters, respectively.

Additional context
As part of this enhancement we could also allow for a kwarg value to be an iterable (collections.abc.Iterable), in which case it would be applied to the request by sending the same parameter multiple times, i.e.:

client.foo(bar=[1,2,3])

would send the request as:

/foo?bar=1&bar=2&bar=3

[berislavlopac]

The issue described under "Additional context" actually only needs the activation of the doseq argument on urlencode in ClientOpenAPIRequest.url. A few tests would certainly be helpful.