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

lib/init-action.js

@@ -160,8 +160,9 @@ async function run() {
 }
 async function getTrapCachingEnabled(featureFlags) {
     const trapCaching = (0, actions_util_1.getOptionalInput)("trap-caching");
-    if (trapCaching !== undefined)
+    if (trapCaching !== undefined) {
         return trapCaching === "true";
+    }
     return await featureFlags.getValue(feature_flags_1.FeatureFlag.TrapCachingEnabled);
 }
 async function runWrapper() {