insights: graphql query schema to support insights based on a search query

Created by: chwarwick

Adds a new query that will eventually support some just in time insights that are based off a search query. This PR adds only the graphql schema and hard codes the result to the QueryInsightsNotAvailable type.

related to https://github.com/sourcegraph/sourcegraph/issues/39222

Test plan

query

query{
  queryInsights(query:"TEST QUERY"){
    __typename
  }
}

result:

{
  "data": {
    "queryInsights": {
      "__typename": "QueryInsightsNotAvailable"
    }
  }
}