Add validation to kdf parameters (#1930)

## Issue Addressed

Closes #1906 
Closes #1907 

## Proposed Changes

- Emits warnings when the KDF parameters are two low.
- Returns errors when the KDF parameters are high enough to pose a potential DoS threat.
- Validates AES IV length is 128 bits, errors if empty, warnings otherwise.

## Additional Info

NIST advice used for PBKDF2 ranges https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf. 
Scrypt ranges are based on the maximum value of the `u32` (i.e 4GB of memory)

The minimum range has been set to anything below the default fields.

crypto/eth2_keystore/src/json_keystore/kdf_module.rs

@@ -4,6 +4,7 @@
 //! data structures. Specifically, there should not be any actual crypto logic in this file.
 
 use super::hex_bytes::HexBytes;
+use crate::DKLEN;
 use hmac::{Hmac, Mac, NewMac};
 use serde::{Deserialize, Serialize};
 use sha2::Sha256;
@@ -128,3 +129,15 @@ pub struct Scrypt {
     pub p: u32,
     pub salt: HexBytes,
 }
+
+impl Scrypt {
+    pub fn default_scrypt(salt: Vec<u8>) -> Self {
+        Self {
+            dklen: DKLEN,
+            n: 262144,
+            p: 1,
+            r: 8,
+            salt: salt.into(),
+        }
+    }
+}