Add api.task_handler and related Client.using_handler

[rochacbruno]

1. Add new handler to extract created/updated resources from task's call_report

Usage examples:

Create a distribution using json_handler

client = Client(cfg, api.json_handler)
spawned_task = client.post(DISTRIBUTION_PATH, body)
# json_handler returns the task call report not the created entity
>>> {'task': ...}
# to have the distribution it is needed to get the task's resources
task_resources = client.get(spawned_task['_href'])
distribution_href = task_resources['created_resources'][0]
distribution = client.get(distribution_href)
>>>  {'_href': ..., 'base_path': ...}

Create a distribution using task_handler

client = Client(cfg, api.task_handler)
distribution = client.post(DISTRIBUTION_PATH, body)
>>> {'_href': ..., 'base_path': ...}
# task_handler resolves the created entity and returns its data

Having an existent client it is possible to use the shortcut::

client.using_handler(api.task_handler).post(DISTRIBUTION_PATH, body)

2. Add syntax-sugar method Client.using_handler

to save few lines of code and some aditional imports when having to use a different handler o a client.

This method clones and injects a new handler dependency in to the
existing client instance and then returns it.

This method is offered just as a 'syntax-sugar' for:

from pulp_smash import api, config

def function(client):  # already receives a client here
    # This function needs to use a different handler
    other_client = api.Client(config.get_config(), other_handler)
    other_client.get(url)

with this method the above can be done in a single line without extra imports:

def function(client):  # already receives a client here
    client.using_handler(other_handler).get(url)

Closes #1178
Ref #1181 (may close it)

[nixocio]

As we talked task_handler it is a pulp 3 only response_handler.

[ragabala]

💯