Rename "repo list" command to "catalog"

The official Docker Registry V2 API calls the commmand to list the repositories
the "catalog", so to keep things as consistent as possible and not invent new
names for things that have meaning in the domain, stick with the same naming
convention.

src/main.rs

@@ -86,7 +86,7 @@ async fn main() -> Result<(), DredgeError> {
     let config = Config::try_from(config_file.as_ref())?;
 
     match args.command {
-        Commands::Repo(repo_args) => commands::repo::handler(&config, &repo_args).await?,
+        Commands::Catalog => commands::catalog::handler(&config).await?,
     }
 
     Ok(())