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 #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:

• [x] 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[bot]]

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 :warning:
cmd/oras/root/discover.go	74.19%	3 Missing and 5 partials :warning:
...md/oras/internal/display/metadata/tree/discover.go	85.18%	2 Missing and 2 partials :warning:
...md/oras/internal/display/metadata/json/discover.go	85.71%	1 Missing and 1 partial :warning:
...ras/internal/display/metadata/template/discover.go	86.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ 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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