Add generic parameter to `AjaxResponse` and `ajax`

Feature Request

Is your feature request related to a problem? Please describe.
ajax(params) returns Observable<AjaxResponse> with field response typed as any.

Describe the solution you'd like
I'd like to be able to pass generic parameter to ajax, like:

ajax<{ field: string }>(params).pipe(map({ response: { field } }) => field))  // properly inferred as `Observable<string>`

Describe alternatives you've considered
Casting type each time, repetitive and annoying.

Start by locating the TypeScript declarations and tests for AjaxResponse and ajax, then inspect how the current response field is typed and inferred. The work is done when callers can provide a generic response shape and the resulting observable infers that shape without repetitive casts, with existing behavior preserved.