Support additional XML API endpoints

Hi Full Story Dev,

From scouring the interwebs, it's my opinion that this is the highest quality open source GCS emulator available today. Thank you!!

### My Goal
For one of my projects, I've leveraging the `https://github.com/ncruces/go-gcp/gmutex` package to do some distributed locking. I'd love to be able to run integration tests against the gcsemu cli to avoid needing credentials and improve test execution time.

### My Problem
However, that package relies on the following Google Cloud Storage XML API endpoints which are not available in this project yet:
* [HEAD Object](https://cloud.google.com/storage/docs/xml-api/head-object)
* Simple [PUT Object](https://cloud.google.com/storage/docs/xml-api/put-object-upload)
* Compose [PUT Object](https://cloud.google.com/storage/docs/xml-api/put-object-compose)

### Proposed Solution
Would you be willing to accept a contribution that added support for these endpoints? I've been hacking on a fork and making some progress.

Since the XML API has some differences in how conditions are passed to the server via query params and a different path structure, I'd probably:
* rename the existing `g.Handler()` -> `g.JsonHandler()` for these routes: https://github.com/fullstorydev/emulators/blob/master/storage/gcsemu/parse.go#L9-L14
* add a new `g.XmlHandler()` for these routes: https://github.com/fullstorydev/emulators/blob/master/storage/gcsemu/parse.go#L16

I'd probably use the test suite for this project and from `gmutex` and examining the docs to validate I've implemented the behavior correctly.

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I understand this is a big ask, and I completely understand if you'd rather decline a contribution of this nature. Let me know if you're willing to consider and discuss. Many thanks for all the excellent work on this project to date!