Add list project environment variable subcommand (one of commands to manage projects)

[threepipes]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
  • Support Project Secrets / Environment Variables #652 is the issue that this PR is targeting
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Changes

=======

• Add a project command
  • Add a subcommand to list project environment variables
    • circleci project secret list <vcs> <org> <project>

Rationale

=========

This PR partially solves #652.
Currently, CircleCI CLI does not have the ability to manage Projects.
This change enables one of these abilities and makes it easier to write the rest subcommands.

Considerations

==============

I will add other subcommands to manage project environment variables like create and delete.

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@c849191). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head d9b35dd differs from pull request most recent head a3f7e7d. Consider uploading reports for the commit a3f7e7d to get more accurate results

Additional details and impacted files

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[threepipes]

@CircleCI-Public/Extensibility Could you review this PR?
As I am new to this project as a contributor, please let me know if there is anything I have to do for your review.

[corinnesollows]

Hello @threepipes This PR is large, could it possibly be broken down into smaller PRs. ie: Accept-Type in one PR and project command in a separate one? Thank you

[threepipes]

@corinnesollows Thank you for your comment and advice.
Sure, I will make a PR for the Accept-Type header.
I will let this PR be as of now because the Accept-Type change is required for the current implementation. So I hope you will check this one after checking the Accept-Type one.

[threepipes]

The header changes are extracted: #784

[threepipes]

@corinnesollows
Thank you for your approval of #784.
This PR no longer contains the Accept-Type header changes.
If there are any more requirements, feel free to let me know.

[flanger001]

It would be extremely tight if this could get some attention @CircleCI-Public/Extensibility @corinnesollows