ClientModel: Pageable abstractions for third-party clients

[annelo-msft]

Overview

System.ClientModel-based client's service methods need to be able to return collections of values from a paged collection, i.e. where subsets of collection elements are returned iteratively over one more calls to the service. TypeSpec is adding pagination support (see overview and microsoft/typespec#705) and in time our generators will generate third-party clients with such service methods from the TypeSpec.

What's in this PR

This PR adds sync and async abstractions for pageable result collections, PageableCollection<T> and AsyncPageableCollection<T>, as well as a ResultPage<T> type that exposes a continuation token and enables enumeration over the results in the page. It also adds a mock client that illustrates standard patterns for implementing these abstractions, as well as tests that illustrate standard usage patterns of the abstractions, e.g. for changing the size of the page requested from the service, and enumerating results using a page's continuation token.

[jsquire]

The approach looks reasonable to me.

It's a shame that the sync/async types are different, but it makes sense if we're trying to align how you enumerate to the sync/async version of the operation that was called.

[jsquire]

Fixing review; request for changes was unintentional and due to a mis-click.