extension_host: Refactor capability checks (#35139)

This PR refactors the extension capability checks to be centralized in
the `CapabilityGranter`.

Release Notes:

- N/A

crates/extension_host/benches/extension_compilation_benchmark.rs

@@ -134,10 +134,12 @@ fn manifest() -> ExtensionManifest {
         slash_commands: BTreeMap::default(),
         indexed_docs_providers: BTreeMap::default(),
         snippets: None,
-        capabilities: vec![ExtensionCapability::ProcessExec {
-            command: "echo".into(),
-            args: vec!["hello!".into()],
-        }],
+        capabilities: vec![ExtensionCapability::ProcessExec(
+            extension::ProcessExecCapability {
+                command: "echo".into(),
+                args: vec!["hello!".into()],
+            },
+        )],
         debug_adapters: Default::default(),
         debug_locators: Default::default(),
     }