Use more privileged memory allocation for the plaintext Buffer in Decrypt

### Problem:
[https://github.com/aws/aws-encryption-sdk-javascript/blob/master/modules/decrypt-node/src/decrypt.ts](url)
```javascript
const plaintext: Buffer[] = []
let messageHeader: MessageHeader | false = false
stream
.once('MessageHeader', (header: MessageHeader) => {
messageHeader = header
})
.on('data', (chunk: Buffer) => plaintext.push(chunk))

// This will check both Uint8Array|Buffer
if (ciphertext instanceof Uint8Array) {
stream.end(ciphertext)
} else if (typeof ciphertext === 'string') {
stream.end(Buffer.from(ciphertext, encoding))
} else if (ciphertext.readable) {
ciphertext.pipe(stream)
} else {
throw new Error('Unsupported ciphertext format')
}

await finishedAsync(stream)
if (!messageHeader) throw new Error('Unknown format')

return {
plaintext: Buffer.concat(plaintext),
messageHeader,
}
```

The `Buffer.concat(plaintext)` will result in a new Buffer. However, the `const plaintext: Buffer[] = []` seem not zero-out where sensitive data might remain in memory until gc.

[//]: # (NOTE: If you believe this might be a security issue, please email aws-security@amazon.com instead of creating a GitHub issue. For more details, see the AWS Vulnerability Reporting Guide: https://aws.amazon.com/security/vulnerability-reporting/ )

Comienza con modules/decrypt-node/src/decrypt.ts e inspecciona cómo se recopilan y combinan los fragmentos de texto plano mediante Buffer.concat. Determina qué significa la asignación privilegiada para esta ruta de descifrado de Node.js y cómo deben manejarse los datos sensibles antes de la recolección de basura. La implementación final debe abordar el problema informado sobre la duración del búfer de texto plano sin cambiar el resultado del descifrado.

Escrito por el modelo de indexación a partir del texto del issue.