azure_core::Error assumes JSON format

### Bug Title

azure_core::Error deserialization is hard-coded to JSON.

### Crate Name

azure_core

### Crate Version

latest

### Description

HTTP error response deserialization assumes JSON format, see [here](https://github.com/Azure/azure-sdk-for-rust/blob/5c7ded236f81f87b568557312ce7bbd17dbd549c/sdk/core/azure_core/src/error/error_response.rs#L201-L203) for an example (also search for `serde_json` in this file).

### Steps to Reproduce

Add a test that returns an azure_core::Error in XML format.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/Azure/azure-sdk-for-rust/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.

Start in sdk/core/azure_core/src/error/error_response.rs at the linked deserialization code and search that file for serde_json usage. Add the XML-format error response test described in the issue, then run the relevant azure_core tests and confirm the test passes without assuming JSON.

Written by the indexing model from the issue text.