.Net: Make some properties of *PromptExecutionSettings classes optional

[SergeyMenshykh]

Context
Today, SK has *PromptExecutionSettings classes with properties that should be optional. Migration to the Azure.AI.OpenAI SDK V2 is a good opportunity to revise the properties of these classes and make them optional if necessary.

To Do

1. Revise all properties of the newly introduced *PromptExecutionSettings classes and change them to optional if necessary.
2. Consider assigning a meaningful defaults to the optional properties:
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