fs: change Config callback into state based callback #3455

This is a very large change which turns the post Config function in
backends into a state based call and response system so that
alternative user interfaces can be added.

The existing config logic has been converted, but it is quite
complicated and folloup commits will likely be needed to fix it!

Follow up commits will add a command line and API based way of using
this configuration system.

backend/tardigrade/fs.go

@@ -41,19 +41,19 @@ func init() {
 		Name:        "tardigrade",
 		Description: "Tardigrade Decentralized Cloud Storage",
 		NewFs:       NewFs,
-		Config: func(ctx context.Context, name string, configMapper configmap.Mapper) error {
-			provider, _ := configMapper.Get(fs.ConfigProvider)
+		Config: func(ctx context.Context, name string, m configmap.Mapper, configIn fs.ConfigIn) (*fs.ConfigOut, error) {
+			provider, _ := m.Get(fs.ConfigProvider)
 
 			config.FileDeleteKey(name, fs.ConfigProvider)
 
 			if provider == newProvider {
-				satelliteString, _ := configMapper.Get("satellite_address")
-				apiKey, _ := configMapper.Get("api_key")
-				passphrase, _ := configMapper.Get("passphrase")
+				satelliteString, _ := m.Get("satellite_address")
+				apiKey, _ := m.Get("api_key")
+				passphrase, _ := m.Get("passphrase")
 
 				// satelliteString contains always default and passphrase can be empty
 				if apiKey == "" {
-					return nil
+					return nil, nil
 				}
 
 				satellite, found := satMap[satelliteString]
@@ -63,23 +63,23 @@ func init() {
 
 				access, err := uplink.RequestAccessWithPassphrase(context.TODO(), satellite, apiKey, passphrase)
 				if err != nil {
-					return errors.Wrap(err, "couldn't create access grant")
+					return nil, errors.Wrap(err, "couldn't create access grant")
 				}
 
 				serializedAccess, err := access.Serialize()
 				if err != nil {
-					return errors.Wrap(err, "couldn't serialize access grant")
+					return nil, errors.Wrap(err, "couldn't serialize access grant")
 				}
-				configMapper.Set("satellite_address", satellite)
-				configMapper.Set("access_grant", serializedAccess)
+				m.Set("satellite_address", satellite)
+				m.Set("access_grant", serializedAccess)
 			} else if provider == existingProvider {
 				config.FileDeleteKey(name, "satellite_address")
 				config.FileDeleteKey(name, "api_key")
 				config.FileDeleteKey(name, "passphrase")
 			} else {
-				return errors.Errorf("invalid provider type: %s", provider)
+				return nil, errors.Errorf("invalid provider type: %s", provider)
 			}
-			return nil
+			return nil, nil
 		},
 		Options: []fs.Option{
 			{