[api-codegen] Add Rust modules override support

[ereOn]

This PR adds the ability to override the generate Rust module during generation, to favor code-reuse and segmentation.

Basically, one can now write:

lgn_api_codegen::generate!(
    Rust,
    "../../apis",
    ["space", "session", "user", "permission"],
    rust_module_mappings => [("../../apis/common.yaml", "lgn_common::api")],
)?;

To force any external references inside ../../apis/common.yaml to use the lgn_common::api crate/module prefix instead of the automatically generated one.