Check for lower case environment variables

[shamsway]

Is your feature request related to a problem? Please describe.

Add a check for lower case environment variables. Other Rubrik SDKs currently specify environment variables for authentication use lower case. On linux/macOS, these variables are case sensitive, and Terraform providers use environment variables that are upper case.

Describe the solution you'd like

Add a check for lower case versions of the environment variables use for authentication, and automatically set upper case versions for use with Terraform.

Describe alternatives you've considered

Do nothing at all. The end user would be responsible for setting the variables properly.

Additional context

While there is no hard standard, historical convention in UNIX is that environment variables are always uppercase with underscores for separation.