console.warn in Impersonated constructor spams warnings

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### A screenshot that you have tested with "Try this API".

N/A

### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://github.com/googleapis/google-auth-library-nodejs/blob/a46981534014a5e6e042d14f0777b62bf559894a/src/auth/impersonated.ts#L124-L126

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

Create a new instance of the `Impersonated` class

### A clear and concise description of what the bug is, and what you expected to happen.

A recent change added a warning when using `Impersonated` class - https://github.com/googleapis/google-auth-library-nodejs/commit/a46981534014a5e6e042d14f0777b62bf559894a

While this is a super useful warning, it creates unnecessary log messages for use cases without security risks.

WDYT about adding a new option to disable it?

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

N/A