add roleSlug for createOrgMembership and fix updateOrgMemberhship

[jameslcarpino]

Description

adds roleSlug param on createOrgMembership. Allows for updateOrgMembership to not require roleSlug.

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.