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/environment/src/lib.rs

@@ -50,6 +50,7 @@ pub struct LoggerConfig {
     pub debug_level: String,
     pub logfile_debug_level: String,
     pub log_format: Option<String>,
+    pub logfile_format: Option<String>,
     pub log_color: bool,
     pub disable_log_timestamp: bool,
     pub max_log_size: u64,
@@ -64,6 +65,7 @@ impl Default for LoggerConfig {
             debug_level: String::from("info"),
             logfile_debug_level: String::from("debug"),
             log_format: None,
+            logfile_format: None,
             log_color: false,
             disable_log_timestamp: false,
             max_log_size: 200,
@@ -252,7 +254,7 @@ impl<E: EthSpec> EnvironmentBuilder<E> {
         let file_logger = FileLoggerBuilder::new(&path)
             .level(logfile_level)
             .channel_size(LOG_CHANNEL_SIZE)
-            .format(match config.log_format.as_deref() {
+            .format(match config.logfile_format.as_deref() {
                 Some("JSON") => Format::Json,
                 _ => Format::default(),
             })