x-ms-meta and x-ms-header-collection-prefix

[RikkiGibson]

When this parameter is specified in Swagger, a Map<String, String> should be provided as a parameter. The runtime should prepend the prefix from Swagger to each map key and add them to request headers before sending.

[RikkiGibson]

@daschult This PR to the C# generator implements x-ms-meta- support. If you search around a bit, particularly in the .cshtml files, you'll find references to the AutoRest model properties which check for the extension. May be useful depending on how we choose to implement this.

https://github.com/Azure/autorest/pull/1744/files