I have to implement an API call to the Salesforce service described here: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/requests_ls_getappointmentcandidates.htm
This is a standard Salesforce rest API, not an Apex class. I found plenty of examples using the connector with Apex classes but none using the standard APIs
Could I use the Mulesoft 4 Salesforce Connector to do that? Which operation is the best suited?
MuleSoft Salesforce connector doesn't let you call specific REST APIs. It provides high level operations that might be implemented by calling REST APIs or some other service. For the REST API that you are pointing to it looks that it is related to Marketing Cloud, so you should look to the MuleSoft Salesforce Marketing Cloud connector documentation.
If you prefer to do the HTTP REST API call directly then you can use the HTTP Connector.