SailPoint jar files are not required for developing a REST api connecting to a target system. Once developed SailPoint can consume them.
There is no particular structure as such, it would be recommended if the API supports pagination, an attribute to represent if the account is enabled/disabled, basic/oauth2 authentication mechanism, get single account api, Create account/group functionality with appropriate response, delete if applicable., these are few which can be performed by SailPoint. You can refer WebService documentation too if you are looking for something specific Integrating SailPoint with Web Services