[MRG] Add convenience function Event.encoded_dataset()

[scaramallion]

Reference issue

Adds a convenience function for accessing the encoded dataset and file meta

Tasks

• Unit tests added that reproduce issue or prove feature is working
• Fix or feature added
• Documentation and examples updated (if relevant)
• Unit tests passing and coverage at 100% after adding fix/feature
• Type annotations updated and passing with mypy

[codecov]

Codecov Report

Merging #888 (f8a1f4d) into main (37388d1) will decrease coverage by 0.15%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #888      +/-   ##
===========================================
- Coverage   100.00%   99.85%   -0.15%     
===========================================
  Files           28       28              
  Lines         8663     8679      +16     
===========================================
+ Hits          8663     8666       +3     
- Misses           0       13      +13     

Files	Coverage Δ
pynetdicom/dsutils.py	100.00% <100.00%> (ø)
pynetdicom/events.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes