Allow setting a proxy

[Wedvich]

I'm currently working on a project for a client where all outbound HTTP calls need to go through a proxy server. I see that an HttpClient is used internally in the sink, but there's no injection point for a HttpMessageHandler anywhere. I suggest another optional parameter in the EventCollector constructor and extension method: proxyAddress, which is exactly what it looks like. This can then be passed in to the EventCollectorClient constructor and be used to create a HttpClientHandler using the provided proxy.

I'd be happy to create a PR for this.

[merbla]

@Wedvich sounds great. Similar to #26 we need to open up HttpMessageHandler

[merbla]

Perhaps similar to the Seq sink? https://github.com/serilog/serilog-sinks-seq/blob/dev/src/Serilog.Sinks.Seq/Sinks/Seq/SeqSink.cs#L53

[merbla]

Great work @Wedvich now to get a release out.