Address proper Boot property management for FunctionCallback interface

[sobychacko]

Options are either users provide FunctionCallback implementations in the application, or we split the current FunctionCallback in such a way that the call() method is separated from the other regular properties.

[ThomasVitale]

Would it be acceptable to remove FunctionCallback as something configurable via application.properties and keep the option to register them as beans?

With the recent changes in the tool calling feature, I wonder how would this work considering the different ToolCallback implementations.

@tzolov what do you think?

References:

• Advancing Tool Support - Part 1 #2064
• Advancing Tool Support - Part 2 #2085
• Advancing Tool Support - Part 3 #2121