fs: Add context to NewFs #3257 #4685

This adds a context.Context parameter to NewFs and related calls.

This is necessary as part of reading config from the context -
backends need to be able to read the global config.

backend/azureblob/azureblob.go

@@ -379,8 +379,7 @@ func (f *Fs) setRoot(root string) {
 }
 
 // NewFs constructs an Fs from the path, container:path
-func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
-	ctx := context.Background()
+func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, error) {
 	// Parse config into Options struct
 	opt := new(Options)
 	err := configstruct.Set(m, opt)