Move environment variable reading to a centralized config location

[rarepolz]

The DURABLE_EXECUTION_TIME_SCALE environment variable is currently read directly in WaitProcessor with os.getenv(). Consider moving this to a centralized configuration class or module to avoid scattered environment variable reads throughout the codebase, especially as more timing-related environment variables (like callback timeouts) may be added in the future.

Originally posted by @bchampp in #133 (comment)