add PHP application template

[endersonmaia]

Depends on cartesi/cli#329

This pull request introduces PHP support to the project, including a new Docker build pipeline, initial PHP application code, and dependency management. The main changes are grouped below:

Build and CI Integration:

• Added php to the list of languages in the GitHub Actions workflow to enable building and testing PHP components.

Docker and Environment Setup:

• Added a php/Dockerfile that builds a deterministic PHP 8.4.11 container for the riscv64 architecture, installs Composer and dependencies, and prepares the runtime environment for Cartesi rollup applications.
• Updated .dockerignore and .gitignore in the php directory to exclude .cartesi and vendor directories from Docker builds and version control.

Dependency Management:

• Introduced a composer.json file in the php directory specifying guzzlehttp/guzzle as a required dependency for HTTP client functionality.

Application Boilerplate:

• Added an initial index.php script that implements a basic event loop for handling Cartesi rollup requests, using Guzzle for HTTP communication and placeholder logic for request handling.