Call Catala plugin backends from custom clerk targets

[denismerigoux]

This will be useful for @rprimet's test GUI editor ; we want a Catala plugin that runs a test and returns results in a VSCode-friendly format. This plugin should receive all the correct includes and context information when invoked with the Catala compiler, so this invocation should be done by clerk, so clerk should be able to call custom backends of the Catala compiler defined in plugins.