Dlc purge

[2shortplanks]

Checklist

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• 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.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

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• Add support for purging DLC from the command line

Rationale

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Users frequently want to purge DLC and ask CircleCI support how they can do so. This adds the functionality to do that from the command line.

Considerations

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This functions by using a "/private" url from dl.circleci.com (which was designed explicitly for this purpose). This is not intended to otherwise be a public endpoint, and is subject to change (though this client will update if that ever happens)

• This client has been designed to determine if the API it's using has been deprecated (i.e. issues a 410 Gone message) and prompt the user to update.
• A new "client" library was added to talk to dl.circlecli.com (similar to the "project" client, etc). This was based off of "rest" like other clients.
• Sensible error messages were added if the user tries to use this on server (which cannot purge DLC in the same manner)

Screenshots

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Here are some helpful tips you can follow when submitting a pull request:

1. Fork the repository and create your branch from main.
2. Run make build in the repository root.
3. If you've fixed a bug or added code that should be tested, add tests!
4. Ensure the test suite passes (make test).
5. The --debug flag is often helpful for debugging HTTP client requests and responses.
6. Format your code with gofmt.
7. Make sure your code lints (make lint). Note: This requires Docker to run inside a local job.

All done (including checking for linting!)