[CT-2036] Parsing: model ‘latest’ attribute

[MichelleArk]

Parse latest: Optional[boolean], as configured in a model yaml property file. Should be available as a model node attribute in the manifest. Should be None for un-versioned models, True if explicitly set via model property, or if model has highest numeric version, and False otherwise.

Validation

• only one version of a given model can have latest: true set
• only versioned models can have latest set to true or false

[jtcohen6]

I'm going to close this as a standalone issue, I think we'll need to capture it as part of the updated version spec proposed in #6866. We can reopen if that's the wrong call