Add brew sponsors command

[jonchang]

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Adds a new hidden developer command, brew sponsors, that queries the GitHub API to get a list of our supporters on GitHub Sponsors.

Ideally this would be automatically updated via continuous integration, but this API requires a personal access token with scopes admin:org and user, which are some pretty big scopes and makes me a bit worried about security. Instead, someone with Owner permissions on the Homebrew org should run this every so often and update the README with its output. Suggestions for automation are welcome.

[MikeMcQuaid]

Nice work!

[MikeMcQuaid]

Instead, someone with Owner permissions on the Homebrew org should run this every so often and update the README with its output.

Maybe someone on the PLC could have this be a recurring reminder?

[jonchang]

Yeah, on the first of each month or thereabouts would be ideal.

[jonchang]

Added a test to please codecov.

[jonchang]

Dunno what codecov wants but I'm just gonna merge this. Thanks for the review!