Add support for configurable HTTP request factory for proxy clients (… #381

Job	Run time
PHP 7.4 Test on Symfony 4.*	26s
PHP 7.3 Test on Symfony	26s
PHP 8.0 Test on Symfony	18s
PHP 7.4 Test on Symfony	22s
PHP 8.1 Test on Symfony jean-beru/fos-http-cache-cloudfront	32s
PHP 8.0 Test on Symfony 6.* dev	19s
PHP 7.3 Test on Symfony 4.4 --prefer-stable --prefer-lowest	18s
PHP 8.0 Test on Symfony 6.*	21s
PHP 7.4 Test on Symfony 5.*	21s
	3m 23s

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