Use Ctrl+C to exit observer in cli mode. (oceanbase#328)

### What problem were solved in this pull request?

Issue Number: close oceanbase#281

Problem: If start observer in cli mode, we cannot exit observer by
Ctrl+C.

### What is changed and how it works?

    在待输入命令时按下Ctrl+C后，注意到日志中有这样的记录：
"[2023-12-04 11:20:30.830780 pid:5338 tid:7fcf688ee7c0 ctx:0 WARN:
my_readline@cli_communicator.cpp:71] >> failed to read line: Interrupted
system call"
    结合“cli_communicator.cpp:71”代码，可以知道此时系统错误被捕捉到且记录在errno中（此时errno为4）。

故当fgets读到空字符串时，若errno==4，可判定此时用户按下了Ctrl+C，这与用户输入“exit”等退出命令是等效的，故可看为用户输入了“interrupted"，借用“bool
is_exit_command(const char
*cmd)”函数判断用户输入的命令是否是退出命令，即可实现类似于用户输入了“exit”等命令的退出效果。

src/observer/net/cli_communicator.cpp

@@ -67,10 +67,16 @@ char *my_readline(const char *prompt)
   fprintf(stdout, "%s", prompt);
   char *s = fgets(buffer, MAX_MEM_BUFFER_SIZE, stdin);
   if (nullptr == s) {
-    free(buffer);
     if (ferror(stdin) || feof(stdin)) {
       LOG_WARN("failed to read line: %s", strerror(errno));
     }
+    /* EINTR(4):Interrupted system call */
+    if (errno == EINTR) {
+      strncpy(buffer, "interrupted", MAX_MEM_BUFFER_SIZE);
+      fprintf(stdout, "\n");
+      return buffer;
+    }
+    free(buffer);
     return nullptr;
   }
   return buffer;
@@ -83,7 +89,8 @@ char *my_readline(const char *prompt)
 */
 bool is_exit_command(const char *cmd)
 {
-  return 0 == strncasecmp("exit", cmd, 4) || 0 == strncasecmp("bye", cmd, 3) || 0 == strncasecmp("\\q", cmd, 2);
+  return 0 == strncasecmp("exit", cmd, 4) || 0 == strncasecmp("bye", cmd, 3) || 0 == strncasecmp("\\q", cmd, 2)
+           || 0 == strncasecmp("interrupted", cmd, 11);
 }
 
 char *read_command()