fix: oras push should return error when artifacttype is empty for spec 1.1

[wangxiaoxuan273]

What this PR does / why we need it:
oras push should return an error when --config is used, --artifact-type is not used and --image-spec is set to v1.1.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1313

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.10%. Comparing base (122f01b) to head (15e944c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1317      +/-   ##
==========================================
+ Coverage   84.06%   84.10%   +0.04%     
==========================================
  Files          90       90              
  Lines        3458     3467       +9     
==========================================
+ Hits         2907     2916       +9     
  Misses        329      329              
  Partials      222      222              

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[wangxiaoxuan273]

There are 3 issues to discuss:

1. Do we allow users to input --image-spec auto? (currently yes)
2. What should be the output of oras push -h for --image-spec? (current display below)

3. We need to document the behavior of auto. (default to v1.1, unless --config is used and --artifact-type is not given)

[shizhMSFT]

LGTM with suggestions

[qweeah]

lgtm