It is possible to authorize a call to the Firebase Remote Config API without using Google API client libraries, but It is not recommended.
As explained in the Oauth 2.0 authorization with service account “your application can complete these tasks either by using the Google APIs client library for your language, or by directly interacting with the OAuth 2.0 system using HTTP. However,the mechanics of 'OAuth 2.0' authentication interactions require applications to create and cryptographically sign JSON Web Tokens (JWTs), and it's easy to make serious errors that can have a severe impact on the security of your application.”
If it is really necessary to avoid the Google API Client Libraries, there are alternative 3rd party tools such as oauth2l. However, Google doesn't officially support this tool and its functionality is not guaranteed.
I just want to highlight the --credentials' argument as the method for selecting your service account JSON file and the cURL command as an example to this use-case.
I recommend you follow the documentation which lays out the steps necessary to authorize the API in question with the Google API Client Library. Bear in mind that the service account JSON file generated by the steps represents the private key for the Firebase Admin SDK service account which is a perfectly appropriate service account to use, but any service account with the Firebase Remote Config roles 'Firebase Remote Config Admin' would also work.
One important topic I would like to emphasise on is that at the moment there’s an ongoing issue with the https://www.googleapis.com/auth/firebase.remoteconfig scope which affects the typical process for authorizing access to this API, You could use the https://www.googleapis.com/auth/cloud-platform scope instead which is mentioned as an alternative in the REST API doc. (This workaround applies if you wish to use "oauth2l")