gtoken request does not use the transporter

[forty]

Environment details

• OS: Linux
• Node.js version: 12
• npm version: Using PNPM 6
• google-auth-library version: 7.14.1

Steps to reproduce

This library uses the gtoken for JWT. The issue is that when using this library, the transporter is not used to do the gaxios request, and as a result, it doesn't work with custom transporter (in our case we use this to customize node http agent).

Given the very small size of gtoken (350 LoC), I think if would make sense to re-implement it here, using the transporter system. But maybe it would be better to also have such transporter mecanism in gtoken so that we could use it here.