Add configuration file handling and move stat method and version there

[valeriupredoi]

Closes #34

Description

• I added two yaml configuration files, one for Posix, another for S3;
• loading of each file is done via a kwarg storage_type passed to Active class
• these files are very basic so far: just "version" and "methods" keys so far; the methods actually work fine for Posix ->
• the method is picked up from the methods key in the config and converted to an actual Python method
• version works fine too

[codecov]

Codecov Report

Base: 86.78% // Head: 87.70% // Increases project coverage by +0.92% 🎉

Coverage data is based on head (8ae7999) compared to base (e933f5b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   86.78%   87.70%   +0.92%     
==========================================
  Files           6        6              
  Lines         401      423      +22     
==========================================
+ Hits          348      371      +23     
+ Misses         53       52       -1     

Impacted Files	Coverage Δ
activestorage/active.py	96.73% <100.00%> (+1.31%)	⬆️

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[markgoddard]

For "production" it would be better to pull in config from a location outside of the Python installation.

[markgoddard]

Perhaps the defaults should live in Python, with YAML providing overrides only?

[markgoddard]

I had a thought about this - as a library (rather than an application), should PyActiveStorage consume a configuration file? Or would it make more sense to pass this information in via Python parameters? For the S3 case, we might consider again a Storage object interface, provided by the application to the library that knows how to connect to S3. Alternatively, each file/S3 object might be represented as a Python object that "knows" how to access it and perform reductions.

We might need some basic configuration for the test cases, which act as an application.