ClientModel: Address APIView feedback regarding Freeze APIs

[annelo-msft]

Address @tg-msft's questions:

• What's the reason for the new Freeze methods on PipelineOptions and RequestOptions?
• How do we expect people to use it?
• What happens when I modify a frozen value?
• How deep does freezing go (i.e., do the allow lists in DiagnosticOptions become immutable too)?
• How does it impact the story for ClientOptions.Default?

Notes from @tg-msft:

https://github.com/Azure/azure-sdk-pr/issues/693 has the details for why ClientOptions.Default was added (tl;dr so Chaos could add PerTransport policies globally and without instrumenting/reshipping all our libraries).

[annelo-msft]

I think we will not end up adding the Freeze API to ClientOptions or have it inherit from ClientPipelineOptions. Moving this to the backlog for now to preserve the questions in case we pick up this piece in the Azure.Core 2.0 integration work.