Add exposed headers

[perkyguy]

As can be seen in #334, there is a weird issue when trying to expose headers and the origin request header is present. This is probably due to something going on with hapi (not quite sure where though). This PR adds a CLI option, corsExposedHeaders to allow general setting of this value.

[ossareh]

@perkyguy I was recently caught out by needing this. It would be great if you could fix the conflict that is being reported and hopefully then @dherault could merge this in?

From my point of view this is pretty critical: this is a major difference between production lambda and serverless-offline lambda. Hapi doesn't actually respect the fact that this header is set - it simply overwrites whatever is there; IMHO it would be more correct to take what's there and then append their headers.

I fully respect #304, and I'd happily help out - except this is already done and just needs a merge and a version publish :)

[perkyguy]

I believe I now have the power to do the merge, maybe not the publish though. However, I'm looking for someone to review this first. @dherault, I'm not sure if that's the protocol or not (or if I'm mistaken about my granted privilages), so let me know.

[dherault]

Yes @perkyguy you could have merged it :) v3.22.0