feat: make --identity-token an alias of --password

[wangxiaoxuan273]

What this PR does / why we need it:

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 #742

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?

[wangxiaoxuan273]

Current display:

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.31373% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 84.49%. Comparing base (bb3443d) to head (46bfbe3).

Files	Patch %	Lines
cmd/oras/internal/option/remote.go	89.74%	3 Missing and 1 partial :warning:
cmd/oras/root/login.go	42.85%	0 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1294 +/- ## ========================================== + Coverage 84.40% 84.49% +0.08% ========================================== Files 101 101 Lines 3617 3637 +20 ========================================== + Hits 3053 3073 +20 Misses 335 335 Partials 229 229 ```

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[qweeah]

-u xxx --identity-token xxx usage should be banned, see https://github.com/oras-project/oras/issues/742#issuecomment-1398076477 <= just find this has already been resolved, good catch.

[TerryHowe]

Closes: https://github.com/oras-project/oras/pull/1040

[shizhMSFT]

LGTM, created #1301 as a follow up so we can make the flag confliction check easier.

Can we resolve #1301 first and then this PR since it really simplifies the code a lot?