Cherry pick - release 1.2: Iotedge check proxy-settings (#5581)

[marianan]

This change introduces a new check in iotedge check called proxy-settings. The new check verifies consistent proxy settings in IoT Edge daemon, IoT Edge agent and Moby daemon.

The following unit tests are added:

• test check::tests::proxy_settings_all_set_should_succeed_test ... ok
• test check::tests::proxy_settings_iot_edge_agent_not_set_should_fail_test ... ok
• test check::tests::proxy_settings_iot_edge_deamon_not_set_should_fail_test ... ok
• test check::tests::proxy_settings_identity_deamon_not_set_should_fail_test ... ok
• test check::tests::proxy_settings_mismatching_values_should_fail_test ... ok
• test check::tests::proxy_settings_moby_deamon_not_set_should_fail_test ... ok
• test check::tests::proxy_settings_none_set_should_succeed_test ... ok

The new functionality is tested:

• on a proxy setup following this automated setup - Azure-Samples/iot-edge-for-iiot: Learn how to use a hierarchy of Azure IoT Edge devices in a manufacturing environment to extract data from industrial assets and upload it to the Cloud while meeting the strict requirements of the Purdue network model. (github.com)
• on an edge device without any proxy settings

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[arsing]

@marianan This sudo invocation also ends up being called by tests, which means these devs cannot run tests unless they allow elevation without prompting.

Please make a PR to fix this. AFAICT no distro should require running that command with sudo to begin with.