Proxy Tests Fail due to invalid Proxy Server

[COM8]

Description

Currently our proxy CI runs fail sometimes in case they can not establish a connection to the test proxy server we are using.

Possible Fix

Add an option CPR_ENABLE_PROXY_TESTS or something like that. When enabled, it will run the proxy tests.
For the CI, we can use a squid_proxy based docker image, for a fast and easy proxy server setup.

Steps to Reproduce

$ git clone https://github.com/libcpr/cpr.git
$ cd cpr && mkdir build && cd build
$ cmake .. -DCPR_BUILD_TESTS=ON
$ cmake --build .
$ ./bin/proxy_tests