Michael Van Canneyt, there is a discussion about using fphttpclient with Google API service account on the forums:
https://forum.lazarus.freepascal.org/index.php/topic,51478.0.html

The last post has a code example of signing a JWT: https://forum.lazarus.freepascal.org/index.php/topic,51478.msg381426.html#msg381426

Is that basically what is needed to get it working?

This is a very limited example of how to do it, yes.

If you wonder why I write 'limited', then check the amount of available encodings:

https://jwt.io/

The example does 1 kind of signing, hardcoded.
However, the server must be able to understand your encoding. If it does not, you're stuck.

So we need a framework where the encoding to be used can be specified.

And that is is what is planned.

Michael.