docs: change examples to use pub APIs

[brianquinlan]

The books API previously used by the package:http, package:cronet_http, package:ok_http, and package:cupertino_http examples never has available quota so the examples appear broken.

Switch the examples to use the Dart pub API.

The Flutter example in package:flutter_http_example is significantly more complex and looks like this:

The examples for the other packages are fairly simple.

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