Pause Ads: Add Support Pause-Ad In A Linear Format in VAST

[Nfrederiksen]

ℹ️ We currently have support for non-linear pause-ads in the Test AdServer.
However, some Ad Servers serve pause ads as linear ads as well. It's basically a normal VAST response but with a MediaFile URL pointing at an image rather than a video. It would be good for the Test AdServer to also be able to return a linear pause ad to emulate other Ad Server that do this.

I'm thinking we could add a query parameter on the existing pause-ad endpoint that specifies if it should be linear or non-linear.

Example pause ad VAST XML:

<VAST version="4.1" xsi:noNamespaceSchemaLocation="vast.xsd">
	<Ad id="83076144.139782267602176" sequence="1">
		<InLine>
			<AdSystem>FreeWheel</AdSystem>
			<AdTitle>Joakim Lundell: Ett Övergivet Barn</AdTitle>
			<Error>https://some.tracking.link/error</Error>
			<Impression id="FWi_83076144.0.1">https://some.tracking.link/impression/tracker.png</Impression>
			<Creatives>
				<Creative AdID="83076144" id="206907629">
					<Linear>
						<Duration>00:00:20</Duration>
						<TrackingEvents>
							<Tracking event="complete">https://some.tracking.link/complete</Tracking>
							<Tracking event="firstQuartile">https://some.tracking.link/1qtl</Tracking>
							<Tracking event="midpoint">https://some.tracking.link/2qtl</Tracking>
							<Tracking event="thirdQuartile">https://some.tracking.link/3qtl</Tracking>
						</TrackingEvents>
						<VideoClicks>
							<ClickThrough id="FWc_8">https://some.tracking.link/click?cr=https%3A//some.ad.link/site</ClickThrough>
						</VideoClicks>
						<MediaFiles>
							<MediaFile delivery="progressive" height="640" id="813784251" type="image/png" width="1135">
                                                                https://s1.fwmrm.net/my_pause_ad.png
                                                        </MediaFile>
						</MediaFiles>
					</Linear>
				</Creative>
			</Creatives>
		</InLine>
	</Ad>
</VAST>

NOTE: As seen in the example we have a duration. The duration in the linear pause-ad in often ignored in the players, but it doesn't hurt to have it as a query parameter.