fix: oras-go client should fallback to docker config if no credentials specified

[tonyay163]

I'm using #17279 to authenticate to Google Artifact Registry as a helm registry using workload identity. Unlike #10218, the other solution does not require installing ESO. This works in general since helm template can get creds via the $HOME/.docker/config.json. However, if you need to use targetRevision: * with your application, this ends up using the oras-go client here which is only configured for static credentials.

This change adds a backwards compatible change where if both the username and password is not configured, it will use the oras-go credentials package to get the docker config from the environment and use that to get credentials.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[tonyay163]

I tested this ETE and it seems to work well

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.81%. Comparing base (864917b) to head (912294f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18133      +/-   ##
==========================================
- Coverage   55.82%   55.81%   -0.02%     
==========================================
  Files         320      320              
  Lines       44431    44436       +5     
==========================================
- Hits        24805    24801       -4     
- Misses      17060    17067       +7     
- Partials     2566     2568       +2     

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[tonyay163]

@crenshaw-dev / @blakepettersson, sorry to ping directly but it looks like you've contributed to this file recently. I have a pretty simple fix here that I've been running for months now and would like to get it merged. It seems like too small of a fix for an enhancement proposal.