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/hash/mod.rs

@@ -1,10 +1,8 @@
-mod base64;
 mod generic_digest;
 mod hash_;
 mod md5;
 mod sha256;
 
-pub use self::base64::Base64;
 pub use self::hash_::Hash;
 pub use self::md5::HashMd5;
 pub use self::sha256::HashSha256;