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

When generating the [front matter for readme pages](https://github.com/Azure/azure-sdk-tools/blob/main/eng/common/scripts/Update-DocsMsMetadata.ps1#L133), 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

Start in eng/common/scripts/Update-DocsMsMetadata.ps1 around the front-matter generation at line 133, and trace how the MSDocService field is read and formatted. Update the metadata handling to support the optional service separator described in the issue, then verify that generated README front matter preserves service-only values and emits service|sub-service values.

Written by the indexing model from the issue text.