[7.x] Add assertSentCount HTTP client assertion

[christophrumpel]

This PR adds a new HTTP client assertion called assertSentCount.

Next to the given assertions, it is also useful to count the recorded requests. This way you can make sure your code only sends x amount of requests and you do not have to be more specific how they look like. Here is an example:

$this->factory->fake();

$this->factory->assertSentCount(0);

$this->factory->post('http://foo.com/form', [
       'name' => 'Taylor',
]);

$this->factory->assertSentCount(1);

$this->factory->post('http://foo.com/form', [
       'name' => 'Jim',
]);

$this->factory->assertSentCount(2);