Cache validated checksums for later executions

The most common case for validation will be that the wrapper jars are unchanged
from a previous workflow run. In this case, we cache the validated wrapper
checksums to minimise the work required on a subsequent run.

Fixes #172

sources/src/wrapper-validation/cache.ts

@@ -0,0 +1,26 @@
+import fs from 'fs'
+import path from 'path'
+import {ACTION_METADATA_DIR} from '../configuration'
+
+export class ChecksumCache {
+    private readonly cacheFile: string
+
+    constructor(gradleUserHome: string) {
+        this.cacheFile = path.resolve(gradleUserHome, ACTION_METADATA_DIR, 'valid-wrappers.json')
+    }
+
+    load(): string[] {
+        // Load previously validated checksums saved in Gradle User Home
+        if (fs.existsSync(this.cacheFile)) {
+            return JSON.parse(fs.readFileSync(this.cacheFile, 'utf-8'))
+        }
+        return []
+    }
+
+    save(checksums: string[]): void {
+        const uniqueChecksums = [...new Set(checksums)]
+        // Save validated checksums to Gradle User Home
+        fs.mkdirSync(path.dirname(this.cacheFile), {recursive: true})
+        fs.writeFileSync(this.cacheFile, JSON.stringify(uniqueChecksums))
+    }
+}