chore(ux): improve error output for unknown spec flag

[qweeah]

What this PR does / why we need it:
This PR improves error output of invalid spec option and fixes #1217

• The parsing of --image-spec and --distribution-spec were refactored into native pflag parsing
• changed the output of invalid value for both flags to

  $ oras attach --distribution-spec 123
  Error: invalid argument "123" for "--distribution-spec" flag: unknown distribution 
  specification flag: 123
  Available options: v1.1-referrers-tag, v1.1-referrers-api
  $ oras push --image-spec 123      
  Error: invalid argument "123" for "--image-spec" flag: unknown image specification flag: 123
  Available options: v1.1, v1.0

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c1a3185) 79.89% compared to head (c12f05c) 80.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1221      +/-   ##
==========================================
+ Coverage   79.89%   80.10%   +0.21%     
==========================================
  Files          67       67              
  Lines        3695     3719      +24     
==========================================
+ Hits         2952     2979      +27     
+ Misses        524      522       -2     
+ Partials      219      218       -1     

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[shizhMSFT]

LGTM