Separate the instantiation and initialisation of the stores

Add a new `init()` method to the `Store` class that is supposed to
handle all the effectful initialisation needed to set-up the store.
The constructor should remain side-effect free and just initialize the
c++ data structure.

The goal behind that is that we can create “dummy” instances of each
store to query static properties about it (the parameters it accepts for
example)

src/libstore/s3-binary-cache-store.cc

@@ -192,6 +192,9 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
 
     S3Helper s3Helper;
 
+    S3BinaryCacheStoreImpl(const Params & params)
+        : S3BinaryCacheStoreImpl("dummy-bucket", params) {}
+
     S3BinaryCacheStoreImpl(
         const std::string & bucketName,
         const Params & params)