Add infra to generate codesamples

[svix-mman]

Added a new function

pub async fn generate_codesamples(
    openapi_spec: &str,
    templates: CodesampleTemplates,
    excluded_operation_ids: BTreeSet<String>,
) -> anyhow::Result<BTreeMap<CodegenLanguage, Vec<CodeSample>>> {
}

Takes in an openapi spec, and returns a Map of language to a vec of code samples.

I moved the content of main.rs to lib.rs.
part of: https://github.com/svix/monorepo-private/issues/11473