zsh: support extra settings in oh-my-zsh plugins (#1106)

Co-Authored-By: Robert Helgesson <robert@rycee.net>

modules/programs/zsh.nix

@@ -152,6 +152,17 @@ let
           Name of the theme to be used by oh-my-zsh.
         '';
       };
+
+      extraConfig = mkOption {
+        default = "";
+        example = ''
+          zstyle :omz:plugins:ssh-agent identities id_rsa id_rsa2 id_github
+        '';
+        type = types.lines;
+        description = ''
+          Extra settings for plugins.
+        '';
+      };
     };
   };
 
@@ -398,6 +409,8 @@ in
         ${envVarsStr}
 
         ${optionalString cfg.oh-my-zsh.enable ''
+            # oh-my-zsh extra settings for plugins
+            ${cfg.oh-my-zsh.extraConfig}
             # oh-my-zsh configuration generated by NixOS
             ${optionalString (cfg.oh-my-zsh.plugins != [])
               "plugins=(${concatStringsSep " " cfg.oh-my-zsh.plugins})"