Add CLI flag to specify the format of logs written to the logfile (#3839)

## Proposed Changes

Decouple the stdout and logfile formats by adding the `--logfile-format` CLI flag.
This behaves identically to the existing `--log-format` flag, but instead will only affect the logs written to the logfile.
The `--log-format` flag will no longer have any effect on the contents of the logfile.

## Additional Info
This avoids being a breaking change by causing `logfile-format` to default to the value of `--log-format` if it is not provided. 
This means that users who were previously relying on being able to use a JSON formatted logfile will be able to continue to use `--log-format JSON`. 

Users who want to use JSON on stdout and default logs in the logfile, will need to pass the following flags: `--log-format JSON --logfile-format DEFAULT`

lighthouse/tests/beacon_node.rs

@@ -1662,7 +1662,24 @@ fn logfile_no_restricted_perms_flag() {
             assert!(config.logger_config.is_restricted == false);
         });
 }
-
+#[test]
+fn logfile_format_default() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert_eq!(config.logger_config.logfile_format, None));
+}
+#[test]
+fn logfile_format_flag() {
+    CommandLineTest::new()
+        .flag("logfile-format", Some("JSON"))
+        .run_with_zero_port()
+        .with_config(|config| {
+            assert_eq!(
+                config.logger_config.logfile_format,
+                Some("JSON".to_string())
+            )
+        });
+}
 #[test]
 fn sync_eth1_chain_default() {
     CommandLineTest::new()