[core] add DPG models

[iscai-msft]

We want to add DPG models to azure.core.serialization.

These models should behave like a mapping, and as a more traditional model. We want these models to be able to replace raw MutableMapping JSON responses that we currently return in DPG generation. This means, when accessed with dict syntax, they should behave exactly the same as a MutableMapping. When accessed with attribute syntax, we will add deserialization logic onto the returned responses. For example, if you access a datetime property with dict syntax, (model["datetime"]), you will get back the datetime string, but when accessed with attribute syntax (model.datetime), you will get back the deserialized datetime object.

We want these models to be easy to read and write, and we want the information needed to create these models to be public knowledge. We used to generate our msrest models with a bunch of hidden dictionaries that were vital to the serialization and deserialization logic. For these new models, we want them to be as above board as possible.

Additionally, one of the main added values of models is to have additional documentation and typehint landing pages. Our models must prioritize excellent documentation and intellisense. When accessing our models with attribute syntax, and creating them with attribute syntax, they should be fully documented with type hinting.

[azure-sdk]

Label prediction was below confidence level 0.6 for Model:ServiceLabels: 'Cosmos:0.21733461,Azure.Core:0.055009324,Cognitive - Form Recognizer:0.054307804'

[github-actions]

Hi @iscai-msft, we deeply appreciate your input into this project. Regrettably, this issue has remained unresolved for over 2 years and inactive for 30 days, leading us to the decision to close it. We've implemented this policy to maintain the relevance of our issue queue and facilitate easier navigation for new contributors. If you still believe this topic requires attention, please feel free to create a new issue, referencing this one. Thank you for your understanding and ongoing support.