How do I authorise access to the Google Calendar API? Edited

[Brendan Gero]

I've been attempting to use the Webhook action to create or update an event via the Google Calendar API. This requires retrieving an access token, which is explained here: https://developers.google.com/identity/protocols/oauth2/service-account#httprest

The problem is that this process requires
1. Creating an object which contains an expiry time and an issue time

2. base64 encoding this object

3. Hashing this object (and other data) with the SHA256 algorithm, to sign the JWT.

Because of the expiry time and issue time, it seems to me that I cannot just determine the base64 encoding and hash once, and hard code the result into the rule - it will need to be done on the fly every time the rule is triggered. My problem is that I'm having trouble finding documentation on how to perform a base64 encoding, and a SHA256 hash, in an automation rule.