Deprecate hash base64, extend decode and add encode commands (#5863)

* feat: deprecate `hash base64` command

* feat: extend `decode` and `encode` command families

This commit
- Adds `encode` command family
- Backports `hash base64` features to `encode base64` and `decode base64` subcommands.
- Refactors code a bit and extends tests for encodings
- `decode base64` returns a binary `Value` (that may be decoded into a string using `decode` command)

* feat: add `--binary(-b)` flag to `decode base64`

Default output type is now string, but binary can be requested using this new flag.

crates/nu-command/src/default_context.rs

@@ -170,6 +170,9 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
             BuildString,
             Char,
             Decode,
+            Encode,
+            DecodeBase64,
+            EncodeBase64,
             DetectColumns,
             Format,
             FileSize,
@@ -382,7 +385,7 @@ pub fn create_default_context(cwd: impl AsRef<Path>) -> EngineState {
             Hash,
             HashMd5::default(),
             HashSha256::default(),
-            Base64,
+            HashBase64,
         };
 
         // Experimental