Decode and Encode hex (#8392)

# Description

I need a command that will transform hex string into bytes and into
other direction.

I've implemented `decode hex` command and `encode hex` command. (Based
on `encode base64` and `decode base64` commands
# User-Facing Changes

```
> '010203' | decode hex
0x[01 02 03]
```

and 

```
> 0x[01 02 0a] | encode hex
'01020A'
```

---------

Co-authored-by: whiteand <andrewbeletskiy@gmail.com>

crates/nu-command/src/default_context.rs

@@ -168,6 +168,8 @@ pub fn create_default_context() -> EngineState {
             Encode,
             DecodeBase64,
             EncodeBase64,
+            DecodeHex,
+            EncodeHex,
             DetectColumns,
             Format,
             FileSize,