weldr: Add support for the v1 API to /projects/source/new

This commit changes the store.PushSource function to take the key as
well as the SourceConfig so that it can be used for v0 or v1.

It adds helper functions for decoding the toml/json into a new
SourceConfig interface type which lets the core source/new code be
shared between the versions.

It also adds tests for the new API behavior.

cmd/osbuild-tests/main_test.go

@@ -112,7 +112,8 @@ func TestSourcesCommands(t *testing.T) {
 	require.NoErrorf(t, err, "Could not create temporary file: %v", err)
 	defer os.Remove(sources_toml.Name())
 
-	_, err = sources_toml.Write([]byte(`name = "osbuild-test-addon-source"
+	_, err = sources_toml.Write([]byte(`id = "osbuild-test-addon-source"
+name = "Testing sources add command"
 url = "file://REPO-PATH"
 type = "yum-baseurl"
 proxy = "https://proxy-url/"