feat!: support showing indirect referrers for all formats of oras discover

[wangxiaoxuan273]

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

1. adds a --depth flag to oras discover command and it applies to all formats. It represents the depth of referrers to display. For example when depth is 1, the command shows only direct referrers; when depth is 2, it shows direct referrers and their referrers. Default is 20, which in practice usually shows all referrers of an image.
2. introduces breaking changes to table and json formats, which previously was showing only the indirect referrers. Now all formats by default show all levels of referrers.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of #1403

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?

[wangxiaoxuan273]

Any ideas about a better default value than 20?

[Wwwsylvia]

If the specified depth is less or equal than 0, it should be considered as infinity. And we can use 0 as the default value.

[wangxiaoxuan273]

If we use 0 to represent infinity, we will need to do extra explanation to the users and it may cause confusion.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 6 lines in your changes missing coverage. Please review.

Project coverage is 84.56%. Comparing base (d971ec5) to head (94d1479).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/oras/internal/display/metadata/model/discover.go	85.71%	2 Missing and 1 partial ⚠️
cmd/oras/root/discover.go	75.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1653      +/-   ##
==========================================
+ Coverage   84.38%   84.56%   +0.18%     
==========================================
  Files         125      125              
  Lines        5661     5657       -4     
==========================================
+ Hits         4777     4784       +7     
+ Misses        629      621       -8     
+ Partials      255      252       -3     

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