Feature: Add an API for giving workers more helpers

**Usecase**

Give users of graphile-worker the ability to pass in custom helpers. This could include custom database connection objects, metadata, application-specific helpers, etc...

For example:

My specific usecase is to pass a Slonik database connection object.

**Implementation**

https://github.com/graphile/worker/blob/1d4fa8c08f10f49eaca2343365a5db2b477206bf/src/helpers.ts#L71

It looks like if [this](https://github.com/graphile/worker/blob/1d4fa8c08f10f49eaca2343365a5db2b477206bf/src/helpers.ts#L71
) TODO was done, this usecase could be satisified.

I'm not sure what the ultimate implementation would look like. Would there be a specific file like `helpers.ts` that would be loaded and named exports would automatically be included?

**Pitfalls/Things to watch out for**

- Naming conflicts with existing helpers -- Is it okay to override?