Enable custom transaction naming (GraphQL specific)

Hi, I've recently started using APM for a django based project using a GraphQL API (built with `graphene_django`). I find it very difficult to view the logs on my service with the current naming convention as they all start with the same prefix (see picture).

The problem is further exasperated by the fact that the naming is based on what the frontend programmer defines. For ex, `GraphQL QUERY Name(value='TradeAssetAmountPickerQuery')`. If I were to change the name query on the frontend to `TradeAssetSomeOtherValuePickerQuery`, it would change the grouping on the dashboard. Given that I am planning on exposing this API publicly, this would make my logs next to impossible to organize based on the fact that developers could name their queries / mutations however they want.

**Describe the solution you'd like**
A hook where I can define my own naming convention for transactions - ideally based on what the mutation / query is actually called in the schema.

**Describe alternatives you've considered**
Forking the library and creating my own build, although I don't really have the time to do this.

Start by tracing how the Django and GraphQL integration names transactions, including the graphene_django request path. Compare the current frontend-derived naming with the schema operation or mutation name described in the issue. Done means users can configure a naming hook for these transactions and the behavior is covered by integration tests.

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