Remove constructors that do not explicitly take a credential parameter

[joseharriaga]

• Removed client constructors that do not explicitly take an API key parameter or an endpoint via an OpenAIClientOptions parameter, making it clearer how to appropriately instantiate a client.
• Removed the endpoint parameter from all client constructors, making it clearer that an alternative endpoint must be specified via the OpenAIClientOptions parameter.
• Removed OpenAIClient's Endpoint protected property.
• Made OpenAIClient's constructor that takes a ClientPipeline parameter protected internal instead of just protected.
• Renamed the User property in applicable Options classes to EndUserId, making its purpose clearer.