ClientModel: Add test coverage for ClientModel types

[annelo-msft]

Foundational pieces

• Allow System.ClientModel test projects to use Azure.Core test framework where appropriate

Root Namespace

• ApiKeyCredential
• BinaryContent
• ClientResult
• ClientResult<T>
• ClientResultException

.Primitives Namespace

• ClientPipelineOptions
• RequestOptions
• ApiKeyAuthenticationPolicy
• ClientPipeline
• ClientRetryPolicy
• HttpClientPipelineTransport
• PipelineMessage
• PipelineMessageClassifier
• PipelinePolicy
• PipelineRequest
• PipelineRequestHeaders
• PipelineResponse
• PipelineResponseHeaders
• PipelineTransport

Other

• Identify if there are internal types we want to add test coverage for
• Move validations in types under ToBeDeleted/ into tests and remove those types