feat: add convertEmptyToNull support for validateArray()

feat: add convertEmptyToNull support for validateArray()

• Added convertEmptyToNull() method to manually enable empty string to null conversion
• Automatically enabled for validate(ServerRequestInterface $request)
• Updated README with Example 3 demonstrating usage

Implement Item and Collection validation features

Added validation helpers: created shortcut functions for validators (alphabetic, alphanumeric, boolean, choice, etc.). Each helper allows easy instantiation of validators with an optional custom message.

initial commit of PHP Validator

[initial commit of PHP Validator