Move and improve the certificates model validation into the shared KV test project

[heaths]

Based on the code for #35254 to validate some of the read-write models, move the code into the shared test project and make some improvements to be more generic and support more types e.g.,

• Rename method to be more obvious e.g., PopulateAndAssertModel<T> c/o @christothes
• Support DateTimeOffsets e.g., use the property name hash code as input to DateTimeOffset.FromUnixTimeSeconds
• Can make code to set a Nullable<T> more generic so those don't all have to be implemented explicitly

This will be specific to Key Vault where models may implement a couple of internal interfaces for serialization but may work for what @m-nash has been thinking about for generic DPG serialization.