Yes, outgoing calls to REST APIS are possible as of 2016.x using "ExecuteWebRequest" (2015.x didn't have this).
Search for "Web Service Methods" in your online help.
Use the "Web Service Connections" under Integration Tools or "Web Service Script (BETA)" Quick Action.
The "Run Web Service" Quick Action only seems to work with WSDLs..
If your desired webservice responds with JSON (they normally do ), have a look at this thread to see how to work with the responses.
Here is the feedback from Product Management:
Q: When can we expect to support Rest API?
A: Fall 2017 most likely.
Q: There are some rumors that Rest API is already built in HEAT SM 2016.2 but hidden, and only available for developers (for testing). Is that true?
A: Yes. This is not documented, and support is not trained, so we are not releasing to customers. Many of our newer integrations are built using REST, so we can validate it is working effectively and we don’t need to change it before giving it to customers.
They are most likely referring to the REST API for Incoming Requests, right?
Both, the quick action and the WebService Connections, are not hidden
I guess PM thought about official documentation and support through HEAT Support Services, in case any issue is found.