First of all, thank you for creating this project.

Since MongoDB Atlas Administration API is moving towards v2, we need to be able to add header with
{ "Accept": "application/vnd.atlas.2025-03-12+json" }
or else, we will always get denied from MongoDB Atlas API with 406.

We can simply add header support for the httpClient so that a user can just include it in their getClient construction. They can change the baseUrl to "https://cloud.mongodb.com/api/atlas/v2", and then add the header like so:

const {user, cluster} = getClient({
  "publicKey": "some public key",
  "privateKey": "some private key",
  "baseUrl": "https://cloud.mongodb.com/api/atlas/v2",
  "projectId": "some project/group id",
  "header": {"Accept": "application/vnd.atlas.2025-03-12+json"}
});

I've made this work on my own project already. Let me know if I can contribute as well.

Thank you!