add option log_callsite to record function callsite (filepath:lineno)

[zzqq2199]

Overview

• Callsite is critical when analyzing complex code paths. For example, when I find the function FA takes a lot of time, I want to find where FA is triggered.
• I add an option log_callsite to record function callsite (filepath:lineno).
• Disabled by default; when enabled, writes callsite into event metadata and shows in the report.

Design & Implementation

• Python API: VizTracer(log_callsite=True).