[Design] Form Recognizer: Copy Model API

[annelo-msft]

Service API details: https://github.com/Azure/azure-rest-api-specs/pull/8349/files

[annelo-msft]

Draft design: https://github.com/annelo-msft/azure-sdk-for-net/blob/1d9eeffaf08a989afb95a93b0cfc3de556c3d82f/sdk/formrecognizer/Azure.AI.FormRecognizer/src/CustomTrainingClient.cs#L328