use go 1.16 new package io/fs

Unify the interface for reading web statics. Now it could read an
OS directory or get the content on live

One could use

//go:embed static
var webFiles embed.FS

anywhere and config dex server to take the file system by setting

WebConfig{WebFS: webFiles}

Signed-off-by: Rui Yang <ruiya@vmware.com>
Co-authored-by: Aidan Oldershaw <aoldershaw@pivotal.io>

Makefile

@@ -24,7 +24,6 @@ build: bin/dex
 
 bin/dex:
 	@mkdir -p bin/
-	@go generate ./...
 	@go install -v -ldflags $(LD_FLAGS) $(REPO_PATH)/cmd/dex
 
 examples: bin/grpc-client bin/example-app