Add CustomCtx type helper

[xixixao]

I'm not sure how to test this exactly in this repo, but I tested it in mine and it works.

In my file with custom functions I can now do:

export const queryFoo = customQuery(
  baseQuery,
  customCtx(async (ctx) => {
    return {
      /// some customization....
    };
  })
);

export type QueryFooCtx = CustomCtx<typeof query>;