chore(sdk): add checkstyle

[cshamrick]

This PR adds Checkstyle into the build process to improve code quality and consistency. Checkstyle was configured using the official sun_checks.xml ruleset (copied to ./checkstyle.xml), with some rules intentionally suppressed (in ./checkstyle-suppressions.xml) to focus on critical areas. The initial scan of the sdk module has revealed 975 violations.

This initial round of violations can be broken down into two main categories:

1. Missing Javadoc Comments (High Volume): The vast majority of the errors are related to missing Javadoc comments for methods and variables, or incorrect Javadoc formatting (e.g., missing @param tags, @return descriptions, etc).
2. Incorrect Visibility Modifiers (High Priority): A significant number of violations are due to variables not being declared as private with appropriate accessor methods. These are important to address as they violate encapsulation principles.

Recommendation:

We should prioritize fixing the "VisibilityModifier" issues first, as they have a direct impact on code design and maintainability. The Javadoc issues, while numerous, can be addressed incrementally.