Provide cache-encryption-key as action input

This makes it easier for users to enable config-cache saving in their workflow.

Config-cache data will only be saved/restored when the key is provided,
and the key is exported as `GRADLE_ENCRYPTION_KEY` for use in subsequent steps.

src/input-params.ts

@@ -29,6 +29,10 @@ export function isCacheCleanupEnabled(): boolean {
     return getBooleanInput('gradle-home-cache-cleanup')
 }
 
+export function getCacheEncryptionKey(): string {
+    return core.getInput('cache-encryption-key')
+}
+
 export function getCacheIncludes(): string[] {
     return core.getMultilineInput('gradle-home-cache-includes')
 }