Respect JSON_SORT_KEYS in _generate_etag

[lafrech]

After all, there is no reason to force this here. Respecting the setting from Flask makes things more consistent with payload dump.

This change will affect users setting JSON_SORT_KEYS to False in that generated ETags will now differ if the order of the dict payload changes. This makes sense for a strong ETag.

There are more things I'd like to clean up in the ETag feature. We're doing too much in flask-smorest and we should rely more on Flask/Werkzeug. This is just an easy first step.

This could be considered a (minor) bug fix. And a (slightly) breaking change.

[codecov]

Codecov Report

Merging #305 (02332d4) into master (6f8a1d7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #305   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files          14       14           
  Lines         793      793           
  Branches      169      169           
=======================================
  Hits          792      792           
  Partials        1        1           

Impacted Files	Coverage Δ
flask_smorest/etag.py	100.00% <100.00%> (ø)

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