feat: initialize azure-acme-provisioner project with core functionality

- Add package.json for project metadata and dependencies
- Implement CLI in src/cli.ts for managing SSL/TLS certificates
- Create Azure Functions host configuration in src/function/host.json
- Set up timer function in src/function/index.ts for scheduled certificate management
- Define configuration loading and error handling in src/lib/config.ts
- Implement DNS zone scanning and challenge management in src/lib/dns.ts
- Develop ACME client for certificate issuance in src/lib/acme.ts
- Create KeyVault store for managing secrets and certificates in src/lib/keyvault.ts
- Implement provisioning logic in src/lib/provisioner.ts for issuing and renewing certificates
- Add TypeScript configuration files for building the project

src/lib/keyvault.ts

@@ -0,0 +1,69 @@
+import { TokenCredential } from '@azure/identity';
+import {
+  CertificateClient,
+  ImportCertificateOptions,
+  KeyVaultCertificateWithPolicy,
+} from '@azure/keyvault-certificates';
+import { SecretClient } from '@azure/keyvault-secrets';
+
+export class KeyVaultStore {
+  private readonly secretClient: SecretClient;
+  private readonly certClient: CertificateClient;
+
+  constructor(credential: TokenCredential, keyVaultUrl: string) {
+    this.secretClient = new SecretClient(keyVaultUrl, credential);
+    this.certClient = new CertificateClient(keyVaultUrl, credential);
+  }
+
+  async getSecret(name: string): Promise<string | undefined> {
+    try {
+      const secret = await this.secretClient.getSecret(name);
+      return secret.value;
+    } catch (err: unknown) {
+      if (isNotFound(err)) return undefined;
+      throw err;
+    }
+  }
+
+  async setSecret(name: string, value: string): Promise<void> {
+    await this.secretClient.setSecret(name, value);
+  }
+
+  async getCertificate(name: string): Promise<KeyVaultCertificateWithPolicy | undefined> {
+    try {
+      return await this.certClient.getCertificate(name);
+    } catch (err: unknown) {
+      if (isNotFound(err)) return undefined;
+      throw err;
+    }
+  }
+
+  async certificateExpiresWithin(name: string, days: number): Promise<boolean | 'missing'> {
+    const cert = await this.getCertificate(name);
+    if (!cert) return 'missing';
+    const expiresOn = cert.properties.expiresOn;
+    if (!expiresOn) return false;
+    const thresholdMs = days * 24 * 60 * 60 * 1000;
+    return expiresOn.getTime() - Date.now() <= thresholdMs;
+  }
+
+  async importCertificate(name: string, pemBundle: string): Promise<void> {
+    const options: ImportCertificateOptions = {
+      policy: {
+        contentType: 'application/x-pem-file',
+        issuerName: 'Unknown',
+        subject: 'CN=unknown',
+      },
+    };
+    await this.certClient.importCertificate(name, Buffer.from(pemBundle), options);
+  }
+}
+
+function isNotFound(err: unknown): boolean {
+  return (
+    typeof err === 'object' &&
+    err !== null &&
+    'statusCode' in err &&
+    (err as { statusCode: number }).statusCode === 404
+  );
+}