.Net: Clean up some things in LiquidPromptTemplate and PromptyKernelExtensions

[stephentoub]

• Liquid template parsing should happen during construction, not on each render
• Liquid prompt template construction should fail for invalid templates
• Default inputs should be evaluated once at Liquid template construction time
• RenderAsync should capture any exceptions into returned Task
• Role regex used in parsing rendered messages should be Compiled
• LiquidPromptTemplateFactory should do arg validation and accomodate a PromptTemplateConfig whose TemplateFormat is null
• Use XML comments instead of normal comments to describe properties in internal DOM
• Remove unnecessary empty primary constructor
• Use a regex to parse the components of a prompty template in order to a) more strictly validate contents but more importantly b) avoid losing part of the template when the separator appears in the contents itself
• Clean up some XML comments
• Set ModelId appropriately for openai
• Avoid storing temperature/top_p in execution settings if they weren't specified
• Add an OutputVariable if the prompty specifies one
• Cache the default template factory rather than creating a new one on each construction

cc: @LittleLittleCloud