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

[threepipes]

Checklist

=========

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have checked for similar issues and haven't found anything relevant.
  • https://github.com/CircleCI-Public/circleci-cli/issues/652 is the issue that this PR is targeting
• [x] This is not a security issue (which should be reported here: https://circleci.com/security/)
• [x] 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>
• Replace a request header Accept-Type with Accept in api/rest/client.go

Rationale

=========

This PR partially solves https://github.com/CircleCI-Public/circleci-cli/issues/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

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

• Replace a request header Accept-Type with Accept in api/rest/client.go
  • Accept header, rather than Accept-Type, is generally used to controll content-type response header
  • The API projects/envvar requires Accept header to return responses of content-type:application/json
    • If not the case, the response header is content-type:text/plain
    • This can't be accepted by the Client of api/rest/client.go
  • With my checking, the runner API doesn't care Accept-Type header
    • I checked that runner resource-class and runner token commands succeeded without Accept-Type or Accept
    • ref. https://github.com/CircleCI-Public/circleci-cli/pull/474 is the PR that the Accept-Type is added

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

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[codecov[bot]]

Codecov Report

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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: https://github.com/CircleCI-Public/circleci-cli/pull/784

[threepipes]

@corinnesollows Thank you for your approval of https://github.com/CircleCI-Public/circleci-cli/pull/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