Add pyinfra equivalent of geerlingguy.composer

[wowi42]

The geerlingguy.composer Ansible role is the #9 most-downloaded role on Ansible Galaxy (~10M downloads). This issue tracks adding a pyinfra equivalent as an example in this repo.

• Ansible Galaxy: https://galaxy.ansible.com/ui/standalone/roles/geerlingguy/composer/
• Source: https://github.com/geerlingguy/ansible-role-composer

Scope

• Port the role's core tasks to a pyinfra deploy / operations
• Install/update Composer (PHP dependency manager)
• Expose equivalent configurable variables (version, global packages, etc.)
• Document usage in a README

cc @wowi42