Allow customization of environment variable names

[averymd]

Summary

It would be nice to be able to change the environment variable names from OP_CONNECT_HOST and/or OP_CONNECT_TOKEN to variables of our choosing when calling new_client_from_environment(). TOKEN is the more important one, since the host can be passed in already.

Use cases

My primary use case is having a single VM that runs multiple applications (not in docker) that need different tokens.

Is there a workaround to accomplish this today?

There are other ways to scope environment env variables. For instance, I could set them in the processes' systemd service files, although then I shift to the problem to having to manage the secrets in those files in my repository and during deployment.