Add FunctionCallContent

[milderhc]

Motivation and Context

Adding a common way to get Function Call contents after a chat completion making use of the KernelContent<T> items that are part of the ChatMessageContent. All function calls of a message can be requested via FunctionCallContent.getFunctionCalls(ChatMessageContent<?>).

Deprecating OpenAIFunctionToolCall to promote use of FunctionCallContent instead.

Fixes: #319

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄