[core-paging] Update the return type for "next" method in PagedAsyncIterableIterator

[HarshaNalluru]

Issue

done is a required property in the IteratorResult<T> if the typescript version is 3.1(for all <3.6 probably).
Whereas done is optional in the return type of the "next()" method in the PagedAsyncIterableIterator.
Compilation with tsc(TS3.1) fails due to this type mismatch when the target version is higher than ES5(example: ES6 or ESNEXT).

src/advanced/listingEntities.ts:40:29 - error TS2322: Type 'PagedAsyncIterableIterator<QueueProperties, EntitiesResponse<QueueProperties>, PageSettings>' is not assignable to type 'AsyncIterable<QueueProperties>'.
  Types of property '[Symbol.asyncIterator]' are incompatible.
    Type '() => PagedAsyncIterableIterator<QueueProperties, EntitiesResponse<QueueProperties>, PageSettings>' is not assignable to type '() => AsyncIterator<QueueProperties>'.
      Type 'PagedAsyncIterableIterator<QueueProperties, EntitiesResponse<QueueProperties>, PageSettings>' is not assignable to type 'AsyncIterator<QueueProperties>'.
        Types of property 'next' are incompatible.
          Type '() => Promise<{ done?: boolean; value: QueueProperties; }>' is not assignable to type '(value?: any) => Promise<IteratorResult<QueueProperties>>'.
            Type 'Promise<{ done?: boolean; value: QueueProperties; }>' is not assignable to type 'Promise<IteratorResult<QueueProperties>>'.
              Type '{ done?: boolean; value: QueueProperties; }' is not assignable to type 'IteratorResult<QueueProperties>'.        
                Property 'done' is optional in type '{ done?: boolean; value: QueueProperties; }' but required in type 'IteratorResult<QueueProperties>'.

 
40   for await (const queue of queues) {
                               ~~~~~~

Fix

Instead of declaring our own return type for the next() method, use IteratorResult<T> to avoid type mismatch.

[xirzec]

Update PR title to reflect this is a core-paging change?

[HarshaNalluru]

More info :

While IteratorResult<T> compiles well with 3.1, 3.6 and 3.9. (value assumes "any" type if TS version>3.5)
IteratorResult<T, T> compiles fine only for TS version > 3.5.

For TS version < 3.6, IteratorResult takes only one argument -> IteratorResult<T>.
For TS version >= 3.6, IteratorResult takes two arguments -> IteratorResult<T, TReturn = any>(the second one is optional).

The only solution is to have different type definitions for lower TS versions.

Created an issue at sandersn/downlevel-dts#40

[HarshaNalluru]

Added separate .dts file for 3.1 to 3.5.

@jeremymeng @bterlson

[jeremymeng]

Looks good to me

[xirzec]

Thanks for fixing this!