Align exp.backoff to current configuration format

[sarroutbi]

The primary reason for this change is to improve code clarity and consistency. Using the full word exponential_backoff instead of the abbreviation expbackoff makes the configuration options easier to understand for new developers and aligns them with the naming conventions used elsewhere in the Keylime codebase. This change has no impact on functionality. To address this, renaming of three configuration variables is performed across the codebase to be more descriptive and consistent:

• expbackoff_initial_delay is now exponential_backoff_initial_delay
• expbackoff_max_retries is now exponential_backoff_max_retries
• expbackoff_max_delay is now exponential_backoff_max_delay

This renaming has been applied consistently in all the places where these parameters are defined or used:

• Configuration Structs (keylime/src/config/base.rs & push_model.rs): The field names in the main AgentConfig struct and the PushModelConfig view have been updated to reflect the new names.

• Agent Logic (keylime-push-model-agent/src/main.rs & registration.rs): All code that accesses these configuration values, either through direct struct access or via getter methods, has been updated to use the new, more descriptive names.