Improvements to cached_result

[Mark90]

• Omit 'self' argument from cache key generation -> This prevents memory addresses in generated keys when decorator is used on class methods
• Raise warnings for potentially unsafe cache key arguments
• Raise error when used on anything else than a coroutine
• Raise error when no arguments are available for cache key generation

Relates to #45

[codecov]

Codecov Report

Attention: Patch coverage is 94.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.64%. Comparing base (3036a32) to head (82108a2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nwastdlib/asyncio_cache.py	93.87%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   80.63%   81.64%   +1.00%     
==========================================
  Files          13       13              
  Lines         470      512      +42     
  Branches       63       70       +7     
==========================================
+ Hits          379      418      +39     
- Misses         71       73       +2     
- Partials       20       21       +1     

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