Authentication file option

### Problem

Right now, authentication is through environment variables $MYSQL_USER and $MYSQL_PASSWORD. The user should have more options.

### Solution

Implement an mysql auth file. Open to other ideas/best practices as well.

Start by locating where $MYSQL_USER and $MYSQL_PASSWORD are read and how authentication is passed to the running MySQL server. Clarify the authentication-file format and supported precedence before implementation; done should include documented behavior and tests for the new option alongside the existing environment-variable path.

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