Stats: Add stats purchases endpoint

[kangzj]

Partially Fixes https://github.com/Automattic/red-team/issues/58

Proposed changes:

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'