Add getService methods to StripeClient and AbstractServiceFactory to allow mocking

[pakrym-stripe]

Mocking magic methods is problematic. Add named methods that customers can override.

$mock = Mockery::mock(\Stripe\StripeClient::class);

$terminalMock = Mockery::mock(\Stripe\Service\Terminal\TerminalServiceFactory::class);

$mock->shouldReceive('getService')
    ->with("terminal")
    ->once()
    ->andReturn($terminalMock);

$terminalMock->shouldReceive('getService')
    ->with("readers")
    ->once()
    ->andReturn($reader);

var_dump($mock->terminal->readers);

Fixes #1350.