report: place Upload() on io.systemd.Report.Uploader rather than io.systemd.Report interface

We really want to use io.systemd.Report for the interface
provided by systemd-report itself, not by its backend. hence, rename the
interface that uploading plugins shall implement to
io.systemd.Report.Uploader.

Note that we ideally should have a varlink interface definition for that
interface. if we had, we'd have noticed that earlier.

man/systemd-report.xml

@@ -89,7 +89,7 @@
         <literal>https://</literal> URL is specified with <option>--url=</option>, an HTTP upload will be
         performed to the specified location. Otherwise, any sockets under
         <filename>/run/systemd/metrics-upload/</filename> will be used to call
-        <function>io.systemd.Report.Upload()</function>.</para>
+        <function>io.systemd.Report.Uploader.Upload()</function>.</para>
 
         <para>Match expressions supported by <command>metrics</command> are supported here too.</para>
 

src/report/report-upload.c

@@ -249,7 +249,7 @@ static int upload_collected(Context *context, sd_json_variant *report) {
 
         ssize_t jobs = varlink_execute_directory(
                         REPORT_UPLOAD_DIR,
-                        "io.systemd.Report.Upload",
+                        "io.systemd.Report.Uploader.Upload",
                         params,
                         /* more= */ false,
                         arg_network_timeout_usec,