Add composer support to install pimcore-plugin DynamicDropdown

Hi !

This pull request to add composer support.
I add composer.json which allow you to install your plugin in a Pimcore project with composer.

You just need to add a
- require "thomas-keil/dynamic-dropdown", "dev-master"
or
- require "thomas-keil/dynamic-dropdown", "0.0.9"

I made a tag release v0.0.9 on my last commit to allow the last require.

Cordially,
Benjamin MARROT