feat: add password_creator for dynamic credentials (e.g. RDS IAM auth)

[DolevGabay]

Summary

Implements the feature requested in #139 — adds a password_creator
parameter to Connection to support dynamic or expiring credentials such as
AWS RDS IAM auth tokens.

Changes

• Connection.__init__ now accepts optional password_creator
• connect() calls it before each new connection
• Supports both sync and async callables
• Fully backward-compatible with existing password parameter

Tests

• Updated conftest.py to use password_creator in test fixtures
• All existing tests now run with dynamic password retrieval via password_creator and a static password
• Verified both synchronous and asynchronous password creator functions work correctly

All existing tests pass on MySQL 8.4.