[Enhancement] Support for Node 14

[jgtvares]

Is your feature request related to a problem? Please describe
The lib doesn't support Node 14.
When calling client.dataExtensionRow(...).get(...), the fs library returns an error: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received an instance of Object.
This is the faulty line:

// CacheService.js
_getOrCreateFile() {
    ...
    return this._writeFile({flag: 'wx'}).then(...)
}

Describe the solution you'd like
Update the lib to support Node 14.

Describe alternatives you've considered
Changing the _writeFile method to something like this: _writeFile("wx") works fine!
That's because newer version of fs expects a string for file content.

Additional context
Add any other context or screenshots about the feature request here.

[DougMidgley]

Feel free to check out https://www.npmjs.com/package/sfmc-sdk :)