Add general constructor for SupergraphConfig (#515)

This allows us to create a `SupergraphConfig` using the same types as
its internal properties in order to support building this object from
non-serialized sources.

apollo-federation-types/src/config/supergraph.rs

@@ -20,6 +20,16 @@ pub struct SupergraphConfig {
 }
 
 impl SupergraphConfig {
+    /// Creates a new SupergraphConfig
+    pub fn new(
+        subgraphs: BTreeMap<String, SubgraphConfig>,
+        federation_version: Option<FederationVersion>,
+    ) -> SupergraphConfig {
+        SupergraphConfig {
+            subgraphs,
+            federation_version,
+        }
+    }
     /// Create a new SupergraphConfig from a YAML string in memory.
     pub fn new_from_yaml(yaml: &str) -> ConfigResult<SupergraphConfig> {
         let parsed_config: SupergraphConfig =