Add EventSourceType and parse_event() generator

[charlieparkes]

Represent event source type as EventSourceType instead of using QueueType everywhere. Generate records from event using generator parse_event()

event_source_type = lpipe.EventSourceType.SQS

for encoded_record, decoded_record, event_source in parse_event(event, event_source_type):
    print(decoded_record)

[codecov]

Codecov Report

Merging #30 into master will decrease coverage by 0.14%.
The diff coverage is 89.39%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   93.40%   93.26%   -0.15%     
==========================================
  Files          18       18              
  Lines         728      742      +14     
==========================================
+ Hits          680      692      +12     
- Misses         48       50       +2     

Impacted Files	Coverage Δ
lpipe/pipeline.py	87.92% <88.88%> (-0.08%)	⬇️
lpipe/__init__.py	100.00% <100.00%> (ø)
lpipe/queue.py	100.00% <100.00%> (ø)

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