Add top-K support to runner usage CSV report

What does this MR do and why?

This MR:

• improves the implementation of the alpha-stage Mutations::Ci::Runners::ExportUsage mutation, so that the user can specify the number of projects to include in the report.
• adds a final <Other projects> line containing the aggregations for the projects that didn't make it to the list.

EE: true

Closes #426562 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Start the clickhouse server ./clickhouse server

2. Run migrations bundle exec rake gitlab:clickhouse:migrate

3. Run mutation in http://gdk.test:3000/-/graphql-explorer:

   Query:

   mutation runnersExportUsage($input: RunnersExportUsageInput!) {
     runnersExportUsage(input: $input) {
       errors
     }
   }

   Query variables:

   {
     "input": {
     	"type": "INSTANCE_TYPE",
       "topK": 10
   	}
   }

   This will dispatch the worker asynchronously in Sidekiq, and the email should be available shortly in http://gdk.test:3000/rails/letter_opener/.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.