Enable gzip responses for application/xml

In the functions applicationhost.config file, `application/xml` is not listed under the `gzip` scheme -- we've got `text/*` and a bunch of `application/...`, but not xml. We've had a customer request to support this -- is it something we can add to our xdt transform for the site extension? Should we make it configurable with some middleware?

Inspect the functions applicationhost.config file and the site extension's XDT transform to see how gzip schemes are defined. Determine whether adding application/xml is sufficient or whether the requested configurability requires middleware; done means the supported approach is documented and XML responses are compressed as intended.

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