Add support for ms.subservice to Update-DocsMsMetadata.ps1

[danieljurek]

When generating the front matter for readme pages, we should support an entry for ms.subservice in our metadata. This can be accomplished by re-using the MSDocService field and using an optional separator. Something like:

[service][|sub-service]

Discovered this requirement in https://github.com/MicrosoftDocs/azure-docs-sdk-python/pull/1717/files