feat: add single request parameter support for RestTemplate

[przemektrzaskala]

Summary

Adds useSingleRequestParameter support to the Java RestTemplate generator.

When enabled, BOAT generates a static *Param inner class and corresponding overloads for operations with multiple parameters. Existing positional-argument methods are preserved for backward compatibility.

Changes

• Add RestTemplate template support for useSingleRequestParameter
• Generate *Param classes for multi-parameter operations
• Keep existing positional overloads unchanged
• Do not generate parameter objects for operations with zero or one parameter
• Add regression tests for enabled and disabled configurations
• Document the option in the Maven plugin README

Example configuration:

<configOptions>
    <useSingleRequestParameter>true</useSingleRequestParameter>
</configOptions>

Testing

• mvn clean verify -Dgpg.skip
• Validated the generated RestTemplate client with the option enabled and disabled
• Verified that existing positional methods remain unchanged
• Verified that *Param overloads are generated for multi-parameter operations
• Verified that single-parameter operations remain unchanged