feat: make all go-template key lower-cased

[qweeah]

What this PR does / why we need it:
This PR makes all formattable names of the metadata output lower cased.

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 #1342

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

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@cc74e50). Click here to learn what that means.

❗ Current head 72a04da differs from pull request most recent head d743e72. Consider uploading reports for the commit d743e72 to get more accurate results

Files	Patch %	Lines
cmd/oras/internal/display/utils/json.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1343   +/-   ##
=======================================
  Coverage        ?   84.50%           
=======================================
  Files           ?      103           
  Lines           ?     3653           
  Branches        ?        0           
=======================================
  Hits            ?     3087           
  Misses          ?      337           
  Partials        ?      229           

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

[shizhMSFT]

LGTM