Create Sentry ownership rules

[andreoliwa]

Problem

Sentry has a feature called Issue Ownership Rules, but it doesn't use a CODEOWNERS file.
Rules need to be manually added in a special Sentry format.

Suggestions

• CLI command cdown sentry
• Read owners from CODEOWNERS file
• Print on stdout using the format expected by Sentry

Don't use Sentry's API for now (I don't even know if it's possible to set rules via API).
The developer should then copy the output and paste on Sentry's text area for rules.