Make 'path' an optional argument for Queue() so that messages may be sent to non-lpipe lambdas

[charlieparkes]

Previously, queue required that the path variable be provided. When used by a path/payload , pipeline.execute_payload would send an lpipe-ready message to the subsequent queue.

my_queue = Queue(
    type=QueueType.SQS,
    url=my_queue_url,
    path="EXAMPLE_PATH"
)

def my_function(**kwargs):
    return Payload(
        path=my_queue,
        kwargs={"foo": "bar"}
    )

# lpipe will send the following message...
{"path": "EXAMPLE_PATH", "kwargs": {"foo": "bar"}}

This MR allows you to send kwargs at the root level of the message, which is especially useful if you want to send messages to a queue read by a service which doesn't use lpipe.

my_queue = Queue(
    type=QueueType.SQS,
    url=my_queue_url,
    # path="EXAMPLE_PATH" <-- no longer a required argument
)

# When used in a payload as demonstrated above, the following message will be sent...
{"foo": "bar"}

[codecov]

Codecov Report

Merging #22 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   93.52%   93.55%   +0.02%     
==========================================
  Files          18       18              
  Lines         695      698       +3     
==========================================
+ Hits          650      653       +3     
  Misses         45       45              

Impacted Files	Coverage Δ
lpipe/contrib/__init__.py	100.00% <ø> (ø)
lpipe/queue.py	100.00% <ø> (ø)
lpipe/pipeline.py	86.78% <100.00%> (+0.17%)	⬆️

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