Add tooltip field to CustomizableDashboardPanel GraphQL API

What does this MR do and why?

Adds tooltip field to the CustomizableDashboardPanel GraphQL API and the analytics dashboard schema to allow users to define a panel's tooltip directly in a dashboard's YAML config file.

In a subsequent MR, this new field will be exposed to the frontend

References

#550270 (closed)

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Related to #550270 (closed)