fix: check for multiple auth parameters

[lechnerc77]

Purpose

• Fix for the situation if username/password as well as ID Token is provided via ENV variables
• Closes [BUG] BTP_IDTOKEN silently takes precedence even when username and password are provided #853

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[X] Bugfix
[ ] Feature
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Test the code via automated test

go test ./...

• Additional test steps

Provide username/password as well as the ID Token (random value) via ENV variables
Execute a terraform plan

What to Check

Verify that the following are valid:

• Automated tests are executed successfully
• An error message is raised when the additional test step is executed

Other Information

n/a

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• The PR is assigned to the Terraform project and a status is set (typically "in review").
• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.

[CHERIANS]

Look good