[Core-http] Support xml namespaces

[joheredi]

OpenAPI supports xml namespace and prefixes (link). However core-http serializer doesn't honor them.

ServiceBus swagger, for example, uses XML namespaces.

Here are some samples on the expected output given a mapper:

Basic type value

const stringMapper = {
  serializedName: "testNode",
  xmlNamespace: "https://microsoft.com",
  type: {
    name: MapperType.String
  }
}

Current

<?xml version="1.0" encoding="UTF-8" ?><testNode>body value</testNode>

Expected

<?xml version="1.0" encoding="UTF-8" ?><testNode xmlns="https://microsoft.com">body value</testNode>

Composite Type

const compositeMapper = {
  required: true,
  serializedName: "bodyArg",
  xmlNamespace: "https://microsoft.com",
  type: {
    name: MapperType.Composite,
    modelProperties: {
      foo: {
        serializedName: "foo",
        xmlNamespace: "https://microsoft.com/foo",
        xmlName: "Foo",
        type: {
          name: "String"
        }
      },
      bar: {
        xmlNamespacePrefix: "bar",
        xmlNamespace: "https://microsoft.com/bar",
        xmlName: "Bar",
        serializedName: "bar",
        type: {
          name: "String"
        }
      }
    }
  }
};

Current

<?xml version="1.0" encoding="UTF-8" ?>
<bodyArg>
   <Foo>Foo</Foo>
   <Bar>Bar</Bar>
</bodyArg>

Expected

<?xml version="1.0" encoding="UTF-8" ?>
<bodyArg xmlns="https://microsoft.com">
   <Foo xmlns="https://microsoft.com/foo">Foo</Foo>
   <Bar xmlns:bar="https://microsoft.com/bar">Bar</Bar>
</bodyArg>

[xirzec]

The changes seem reasonable, though we should think about how this plays with this other pending PR:

#9599

Also maybe @jeremymeng could take a look at this since he's worked in this space before?

[jeremymeng]

The changes seem reasonable, though we should think about how this plays with this other pending PR:

#9599

There might be some merge conflict. Not sure yet about the impact of the char key changing from _ to #

[joheredi]

@jeremymeng, @xirzec I think that the change to # should work fine we'd just need to replace the _ added in this PR for #

I have figured out the gaps in the initial draft and added a bunch of new tests validating the XML and JSON behavior. Please take another look once you get a chance and let me know if you find any potential risks

[xirzec]

I'm happy with these changes, especially the new tests.

Could you also do me a huge favor and copy/paste these changes over to core-xml? https://github.com/Azure/azure-sdk-for-js/tree/master/sdk/core/core-xml

[jeremymeng]

Looks great! Thanks for the test coverage!