[foundation] Fix POST/PUT issues with NSUrlSessionHandler. Fixes #52682 (#1772)

[spouliot]

The latest NSUrlSessionHandler implementation switched from loading the
request into memory (NSData) to loading it from a stream (NSInputStream).
The later is more efficient, at least for large POST'ed requests which
could, in theory, not even fit in memory.

Now using a stream means a different API is used. NSMutableUrlRequest
Body and BodyStream properties are exclusive but also a bit different as
the later won't set Content-Length [1] and switch to chunked encoding [2]

Even if the current code is compliant with HTTP/1.1 this breaks some
common POST/PUT usage that worked "as expected" in the previous (C8)
release.

To fix this we ask for the stream's length. We assume that if the length
is known then it's (likely) already (or fitting) in memory and use the
(old, in memory) Body property, which will set Content-Length.

If the length is unknown then we use the BodyStream approach so large
POST will continue to work (well over past/C8 limits).

Finally there might be case where developers will prefer to avoid the
extra memory (of Body) so a new property MaxInputInMemory is added
to set a maximum. It defaults to Int64.MaxValue so Body will be
used whenever possible. It can be set to 0 to get back the original C9
(always use a stream) behavior.

references:
[1] https://bugzilla.xamarin.com/show_bug.cgi?id=52682
[2] https://bugzilla.xamarin.com/show_bug.cgi?id=52711
[3] https://devforums.apple.com/message/919330#919330

[monojenkins]

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