Add Service Desk email rate limit columns to plan_limits

What does this MR do and why?

Add Service Desk email rate limit columns to plan_limits

Add two plan_limits columns, service_desk_outbound_emails_per_hour and service_desk_outbound_emails_per_day, and seed per-plan values for GitLab.com plans. These will back a rate limiter on outbound Service Desk email, added in a follow-up MR. This MR only adds the schema and seed data, no behavior change.

The default plan is intentionally left unseeded (stays 0, meaning unlimited), since self-managed instances only ever resolve the default plan. This keeps the limits GitLab.com-only.

https://gitlab.com/gitlab-org/gitlab/-/work_items/627182

Changelog: added

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Related to #627182