Add project's shared_runners_duration to GraphQL

What does this MR do and why?

Related to #355666 (closed)

This MR exposes Ci::Minutes::ProjectMonthlyUsage#shared_runners_duration via GraphQL type Ci::Minutes::ProjectMonthlyUsageType the same way as we do it at namespace-level via NamespaceMonthlyUsageType.

This change allows us to show a breakdown of actual shared runners duration by project in the Usage Quota page for a group or user namespace.

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