Add GitHub billing usage plugin

[KingParmenides]

What does this PR change?

• Adds a GitHub billing usage OpenCost plugin for issue Github Billing and Usage #42.
• Supports GitHub enhanced billing usage endpoints for both organizations and users.
• Maps returned usage line items such as Actions, Packages, and storage SKUs into OpenCost custom costs.
• Adds config validation, README usage notes, and mocked API unit tests.
• Adds github to the plugin manifest.

How will this PR impact users?

Users can ingest GitHub billing usage into OpenCost custom costs with a billing-capable GitHub token and account name.

Does this PR address any GitHub or Zendesk issues?

Closes #42

How was this PR tested?

• go test ./... from pkg/plugins/github
• go vet ./... from pkg/plugins/github
• VERSION=dev COMMIT=codex ./tools/build-plugins ./pkg/plugins/github/go.mod
• git diff --check

Bounty

This is a submission for the $1k plugin bounty mentioned on #42. BTC payout address if accepted: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ