feat(metricprovider): credentials to download plugin

[ariadnarouco]

In our company, we have a use case where we need to create a new plugin as a metric provider. For security reasons the plugin needs to be private and cannot be uploaded to a public storage.
Therefore, we have the need to use credentials for downloading the plugin securely.

We have followed k8s secretRef pattern to specify how the credentials are passed.

Additionally, we had an issue with 404 status code. The downloader says that the plugin is downloaded correctly but it’s not true.

Fixes: #3708

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[github-actions]

Published E2E Test Results

  4 files    4 suites   3h 14m 38s ⏱️
113 tests 104 ✅  7 💤 2 ❌
454 runs  424 ✅ 28 💤 2 ❌

For more details on these failures, see this check.

Results for commit 2b091a2.

♻️ This comment has been updated with latest results.

[github-actions]

Published Unit Test Results

2 285 tests   2 284 ✅  2m 59s ⏱️
  128 suites      0 💤
    1 files        1 ❌

For more details on these failures, see this check.

Results for commit 2b091a2.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.94%. Comparing base (53c4f12) to head (ec7937c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3905      +/-   ##
==========================================
+ Coverage   82.66%   82.94%   +0.28%     
==========================================
  Files         163      163              
  Lines       22895    22914      +19     
==========================================
+ Hits        18927    19007      +80     
+ Misses       3092     3032      -60     
+ Partials      876      875       -1     

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Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
29.6% Duplication on New Code

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