Add targeting expression for mobile first run if first_run is selected

[jaredlockhart]

If a user selects the 'first run' checkbox when creating an experiment for a mobile experiment, we should also include a targeting expression which checks that it's specifically the first session. This may involve adding a new targeting parameter to the SDK.

┆Issue is synchronized with this Jira Task

[tarikeshaq]

The attributes for both firefox-ios and fenix are called isFirstRun. They're custom targeting attributes (i.e set by the apps and not by the SDK by default) so our integration tests will probably need to add them as custom targeting attributes too. probably here in the tests

• firefox-ios: https://github.com/mozilla-mobile/firefox-ios/blob/f49de2319c448122608700c14ed5252e6ff2b8a2/Client/Application/AppLaunchUtil.swift#L134
• fenix: https://github.com/mozilla-mobile/fenix/blob/4777f03d9a518c71e5db5eab307e5ec52be69f9e/app/src/main/java/org/mozilla/fenix/experiments/NimbusSetup.kt#L83

note: Focus android and focus iOS do not have those attributes yet

[jaredlockhart]

Amazing thank you @tarikeshaq !