Put builder class inside PortableFunctionCallingOptions

[markpollack]

Currently the FunctionCallingOptionsBuilder is a separate class, it should be removed and instead a 'standard spring-ai' builder pattern be used inside PortableFunctionCallingOptions.

'standard spring-ai' builder at the moment is

• no 'with' prefix on methods
• check for passing in null values
• for a 'simple' case like options, where there aren't going to be multiple implementations, there isn't a need for a builder interface.

Also rename PortableFunctionCallingOptions to DefaultFunctionCallingOptions

[ilayaperumalg]

This is addressed via b25d6e8