refactor: Use cURL's own basic auth implementation

Drops the dependency on the base64-crate, which it turns out isn't
necessary as cURL has this built-in.

Cargo.toml

@@ -9,12 +9,10 @@ license = "GPL-3.0-or-later"
 repository = "https://github.com/tazjin/crimp"
 
 [features]
-default = [ "json", "basic_auth" ]
+default = [ "json" ]
 json = [ "serde", "serde_json"]
-basic_auth = [ "base64" ]
 
 [dependencies]
 curl = "0.4"
 serde = { version = "1.0", optional = true }
 serde_json = { version = "1.0", optional = true }
-base64 = { version = "0.10", optional = true }