Add an API to read external target memory.

ReadExternalTargetMemory reads bytes out of the target's address space:
process_vm_readv for live bytes (gated like ptrace access), falling back
to the target's on-disk image located by scanning /proc/<pid>/maps
directly - the caller runs while the symbol thread may be rebuilding the
shared image cache on refresh.

public/client/TracyCallstack.hpp

@@ -93,6 +93,8 @@ bool InitExternalTarget( pid_t targetPid );
 uint32_t GetExternalTargetPid();
 const char* GetExternalTargetName();
 uint64_t GetExternalTargetExeTime();
+
+size_t ReadExternalTargetMemory( uint64_t addr, uint32_t size, char* buf );
 #endif
 
 #ifdef TRACY_DEBUGINFOD