feat: support --format in oras discover

[qweeah]

What this PR does / why we need it:
This PR

1. Added a new --format flag following spec doc(spec): formatted ORAS CLI output #1199
2. Changes tree as default output of oras discover

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 #638

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 83.89262% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 84.34%. Comparing base (b5217ef) to head (46fd349).

Files	Patch %	Lines
...d/oras/internal/display/metadata/table/discover.go	78.94%	4 Missing and 4 partials ⚠️
cmd/oras/root/discover.go	74.19%	3 Missing and 5 partials ⚠️
...md/oras/internal/display/metadata/tree/discover.go	85.18%	2 Missing and 2 partials ⚠️
...md/oras/internal/display/metadata/json/discover.go	85.71%	1 Missing and 1 partial ⚠️
...ras/internal/display/metadata/template/discover.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1299      +/-   ##
==========================================
- Coverage   84.40%   84.34%   -0.07%     
==========================================
  Files          97      101       +4     
  Lines        3540     3615      +75     
==========================================
+ Hits         2988     3049      +61     
- Misses        329      337       +8     
- Partials      223      229       +6     

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[shizhMSFT]

LGTM with suggestions

[shizhMSFT]

LGTM