Adopt AHC's .shared singleton as the default Client

[czechboy0]

Motivation

As a convenience when you don't need to create a custom AHC Client instance, we provide a singleton client as the default. AHC added their own Client.shared singleton, so let's adopt that and remove the transport's internal singleton, which was used for the same purpose.

Modifications

• Replaced the tranport's internal singleton with the AHC-provided one.
• Use Int64 directly and skip conversions now that AHC also uses Int64 for the body byte size.

Result

• No internal singleton is created now, possibly avoiding having two AHC singletons in a given process.
• Fixed a warning around Int/Int64 conversion.

Test Plan

Unit tests still pass.