feat: add org membership APIs, surface error messages, and url-encode GET parameters

[sxlijin]

This is a lot of code, but it's mostly pretty mechanical. Changes are as follows:

• new files in src/user_management/operations are just support for APIs that workos-rust does not currently have
• error handling now exposes HTTP error bodies, see src/core changes
• get_organization_by_external_id and get_user_by_external_id now urlencode their request params (which allows for example embedding / in an external ID correctly)

Disclaimer: this is largely AI-generated, but I've reviewed all of it. There's one or two unnecessary changes but I don't think there's anything that's API-breaking.