Fix AuctionContext model to be suitable for JSON encoding

[rpanchyk]

Fixes error:

java.lang.IllegalArgumentException: No serializer found for class org.prebid.server.execution.Timeout and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: org.prebid.server.analytics.model.AuctionEvent["auction_context"]->org.prebid.server.auction.model.AuctionContext["timeout"])
        at com.fasterxml.jackson.databind.ObjectMapper.valueToTree(ObjectMapper.java:2969)
        at org.prebid.server.analytics.pubstack.PubstackEventHandler.buffer(PubstackEventHandler.java:104)
        at org.prebid.server.analytics.pubstack.PubstackEventHandler.handle(PubstackEventHandler.java:84)
        at org.prebid.server.analytics.pubstack.PubstackAnalyticsReporter.processEvent(PubstackAnalyticsReporter.java:100)
        at org.prebid.server.analytics.AnalyticsReporterDelegator.processEventByReporter(AnalyticsReporterDelegator.java:193)
        at org.prebid.server.analytics.AnalyticsReporterDelegator.lambda$delegateEvent$2(AnalyticsReporterDelegator.java:97)
        at io.vertx.core.impl.ContextImpl.executeTask(ContextImpl.java:369)
        at io.vertx.core.impl.EventLoopContext.lambda$executeAsync$0(EventLoopContext.java:38)
        at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163)
        at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:404)
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:466)
        at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:897)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.lang.Thread.run(Unknown Source)