Prompt engineering: function signatures

This MR adds the function to extract function signatures in the prompt engine. In a later MR, this function will be added to a dataflow pipeline. As part this MR, more language supports are added, to align with what we have in production.

Specifically, this MR reimplements imports extractor and function signature extractor from ai-gateway to here.

Ref: https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/prompt-library/-/issues/17