Fix naming of validators in CLI (#1332)

## Issue Addressed

NA

## Proposed Changes

- Adds a `lighthouse account validator list` command, to list all known validators.
- Fixes the validator names; previously they were using a full path (e.g., `"/home/paul/.lighthouse/validators/0x8ce25415d078bdc83133758604578ba51707a55965eeca8982f44695db7432d6ff1c23529020a971faa68ab60baf3118"` but now we only use the final directory name (e.g., `0x8ce25415d078bdc83133758604578ba51707a55965eeca8982f44695db7432d6ff1c23529020a971faa68ab60baf3118`).

common/validator_dir/src/manager.rs

@@ -164,10 +164,12 @@ impl Manager {
     ///
     /// Returns an error if a directory is unable to be read.
     pub fn directory_names(&self) -> Result<HashMap<String, PathBuf>, Error> {
-        Ok(HashMap::from_iter(
-            self.iter_dir()?
-                .into_iter()
-                .map(|path| (format!("{:?}", path), path)),
-        ))
+        Ok(HashMap::from_iter(self.iter_dir()?.into_iter().filter_map(
+            |path| {
+                path.file_name()
+                    .and_then(|os_string| os_string.to_str().map(|s| s.to_string()))
+                    .map(|filename| (filename, path))
+            },
+        )))
     }
 }