api: get_streams (#886)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
MSE method of streaming video needs the dimensions up front. This new
api delivers that. Also provides an easier way to determine which
screens are available and a way to discover port number.

## Testing
See pending changes in substream project

src/spice2x/api/stream_format.h

@@ -4,6 +4,8 @@
 #include <functional>
 #include <memory>
 #include <string>
+#include <utility>
+#include <vector>
 
 #include "capture_pump.h"
 
@@ -35,4 +37,7 @@ namespace api {
     // null when the path does not name a format this build supports
     std::unique_ptr<StreamWriter> make_stream_writer(
             const std::string &path, int quality, int fps);
+
+    // name and path of every format compiled into this build, for clients to pick from
+    std::vector<std::pair<std::string, std::string>> stream_formats();
 }