Refactor handling of feature flags

This commit centralizes how feature flags are handled. All feature flags
must now add an entry in the `featureFlagConfig` dictionary. This
dictionary associates the flag with an environment variable name and
optionally a minimum version for CodeQL.

The new logic is:

- if the environment variable is set to false: disabled
- if the minimum version requirement specified and met: disabled
- if the environment variable is set to true: enable
- Otherwise check feature flag enablement from the server

src/testing-utils.ts

@@ -160,3 +160,11 @@ export function mockFeatureFlagApiEndpoint(
 
   sinon.stub(apiClient, "getApiClient").value(() => client);
 }
+
+export function mockCodeQLVersion(version) {
+  return {
+    async getVersion() {
+      return version;
+    },
+  } as CodeQL.CodeQL;
+}