settings_ui: Add maybe settings (#40724)

Closes #ISSUE

Adds a `Maybe<T>` type to `settings_content`, that makes the distinction
between `null` and omitted settings values explicit. This unlocks a few
more settings in the settings UI

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/settings/src/settings_content/terminal.rs

@@ -134,7 +134,19 @@ pub struct TerminalSettingsContent {
 }
 
 /// Shell configuration to open the terminal with.
-#[derive(Clone, Debug, Default, Serialize, Deserialize, PartialEq, Eq, JsonSchema, MergeFrom)]
+#[derive(
+    Clone,
+    Debug,
+    Default,
+    Serialize,
+    Deserialize,
+    PartialEq,
+    Eq,
+    JsonSchema,
+    MergeFrom,
+    strum::EnumDiscriminants,
+)]
+#[strum_discriminants(derive(strum::VariantArray, strum::VariantNames, strum::FromRepr))]
 #[serde(rename_all = "snake_case")]
 pub enum Shell {
     /// Use the system's default terminal configuration in /etc/passwd