Implement packet capture client and models

[niklasberglund]

This PR adds support for packet capture in end to end tests. In BaseUITestCase there are the functions startPacketCapture() and stopPacketCapture() for tests to use for starting and stopping packet capture sessions. The PCAP file for a packet capture session will be attached to the test run in teardown.

Created testLeaks in LeakTests as an example of how to do packet capture. The test is just there as an example right now and is not analysing the captured traffic yet.

To test the changes in this PR out you will need to copy the latest UITests.xcconfig.template file or add this to your UITests.xcconfig file:

// Base URL for the packet capture API. Note that // will be treated as a comment, therefor you need to insert a ${} between the slashes for example http:/${}/8.8.8.8
PACKET_CAPTURE_BASE_URL = http:/${}/8.8.8.8

Also note that tests are now running on staging environment, so you need to connfigure staging accounts for NO_TIME_ACCOUNT_NUMBER and HAS_TIME_ACCOUNT_NUMBER in your UITests.xcconfig.

Try out the changes by running testLeaks. You must be on the app-team-ios-tests WiFi. After running the test you should find a PCAP file for the test run in the test report under tear down.

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This change is 

[linear]

IOS-534 Write a test class that will automatically capture traffic when tests are ran

[niklasberglund]

Added a second attachment to test runs - JSON data. Maybe not something that should be kept later but it's very useful during development to inspect the data tests can see, rather than only inspecting the PCAP file which shows all traffic.

Reviewable status: 0 of 10 files reviewed, all discussions resolved

[acb-mv]

Reviewed 10 of 10 files at r1.
Reviewable status: 8 of 10 files reviewed, all discussions resolved

[niklasberglund]

Reviewable status: 7 of 10 files reviewed, 1 unresolved discussion (waiting on @acb-mv)

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ios/MullvadVPNUITests/Networking/Networking.swift line 53 at r3 (raw file):

                // Check if interface is en0 which is the WiFi connection on the iPhone
                let name = String(cString: interfacePointer.pointee.ifa_name)
                if name.hasPrefix("en") {

Fix for one of the Mullvad iPhones using network interface en2 instead of en0. I was under the impression all iPhones and iOS versions use en0 for the wifi network interface, but turned out other en interfaces can be used.

[mojganii]

it sometimes fails to retrieve file, perhaps there is some problem with IO which needs to be tested more.

Reviewable status: 7 of 10 files reviewed, 1 unresolved discussion (waiting on @acb-mv)

[mojganii]

Reviewable status: 7 of 10 files reviewed, 1 unresolved discussion (waiting on @acb-mv)

[acb-mv]

Reviewed 1 of 2 files at r2, 1 of 1 files at r3, 2 of 2 files at r4, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion