Adding IsRequired property to all parameters

[teresaqhoang]

Motivation and Context

Extending parameter objects to include an IsRequired field for better planner resiliency.

Description

Also, includes some logic changes to Copilot PlanViewer to show INPUT value.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[lemillermicrosoft]

#1195 is proposing some changes in the area. Might use this to help speed up things over there.

[adrianwyatt]

LGTM - but I'm curious about using string.join as well.

[teresaqhoang]

Closing in favor of Stephen's PR: #1195

Will an issue to re-evaluate how we specify required / optional parameters to the Planner. It seems like Stephen's PR will handle native functions well, where all parameters are required but have a DefaultValue of null when optional. But some native functions specify (optional) in the parameter description, whereas we're specifying (required) in the description of rest api parameters when necessary. The AI could get confused for parameters with descriptions that don't contain either postfix, so we should pick one flag or the other for the planner's functionView.