refactor: Move generate content key method to descriptor

[TerryHowe]

What this PR does / why we need it:

Move GeneerateContentKey to descriptor package and increase test coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.33%. Comparing base (f6440c3) to head (5782e3b).

❗ Current head 5782e3b differs from pull request most recent head f56bda4

Please upload reports for the commit f56bda4 to get more accurate results.

Files	Patch %	Lines
cmd/oras/root/pull.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1415   +/-   ##
=======================================
  Coverage   85.33%   85.33%           
=======================================
  Files         107      106    -1     
  Lines        3798     3798           
=======================================
  Hits         3241     3241           
  Misses        333      333           
  Partials      224      224           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[qweeah]

LGTM with minor suggestions.