Support custom extensions type in `graphql-request`

It would be nice to use a custom `extensions` type in `graphql-request` plugin when the `rawRequest` config is enabled.

**Describe the solution you'd like**
A configuration parameter to change the default `any` type to something else.

**Additional context**
To date, `extensions` is hardcoded as `any`, which is the worst possible type for good static typing. `unknown` would be a great replacement, but changing this is a breaking change. Adding a custom configuration in order to let the user decide is probably a good compromise.