iOS/tvOS specific HttpClientHandler support

[marek-safar]

Implement support for native iOS/tvOS specific HttpClientHandlers as described in the design proposal https://github.com/dotnet/designs/blob/main/accepted/2020/mono-convergence/platform-specific-httpclient.md#xamariniosxamarintvosxamarinwatchos

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Implement support for native iOS/tvOS specific HttpClientHandlers as described in the design proposal https://github.com/dotnet/designs/blob/main/accepted/2020/mono-convergence/platform-specific-httpclient.md#xamariniosxamarintvosxamarinwatchos

Author:	marek-safar
Assignees:	-
Labels:	area-System.Net.Http, os-ios, os-tvos
Milestone:	6.0.0