Mnemonic key recovery (#1579)

## Issue Addressed

N/A

## Proposed Changes

Add a  `lighthouse am wallet recover` command that recreates a wallet from a mnemonic but no validator keys.  Add a `lighthouse am validator recover` command which would directly create keys from a mnemonic for a given index and count.

## Additional Info


Co-authored-by: Paul Hauner <paul@paulhauner.com>

common/account_utils/src/lib.rs

@@ -107,6 +107,23 @@ pub fn read_password_from_user(use_stdin: bool) -> Result<ZeroizeString, String>
     result.map(ZeroizeString::from)
 }
 
+/// Reads a mnemonic phrase from TTY or stdin if `use_stdin == true`.
+pub fn read_mnemonic_from_user(use_stdin: bool) -> Result<String, String> {
+    let mut input = String::new();
+    if use_stdin {
+        io::stdin()
+            .read_line(&mut input)
+            .map_err(|e| format!("Error reading from stdin: {}", e))?;
+    } else {
+        let tty = File::open("/dev/tty").map_err(|e| format!("Error opening tty: {}", e))?;
+        let mut buf_reader = io::BufReader::new(tty);
+        buf_reader
+            .read_line(&mut input)
+            .map_err(|e| format!("Error reading from tty: {}", e))?;
+    }
+    Ok(input)
+}
+
 /// Provides a new-type wrapper around `String` that is zeroized on `Drop`.
 ///
 /// Useful for ensuring that password memory is zeroed-out on drop.