feat: add StaticFileFS

[thinkgos]

I was miising feature of Serving file form http.FileSystem
there has StaticFile,Static,StaticFS method, since go1.16 embed feature, I am not find how to get a file from embed fs. follow is my want

//go:embed mypic.png
var pic embed.FS

func main() {
	r := gin.Default()
        r.StaticFileFS("/alias_pic.png", "mypic.png", http.FS(pic))
        
	err := r.Run(":9000")
	if err != nil {
		panic(err)
	}
}

Use cases:
- Serve file by alias from http.FileSystem

[codecov]

Codecov Report

Merging #2749 (2a24581) into master (34ce210) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2749      +/-   ##
==========================================
+ Coverage   98.69%   98.75%   +0.06%     
==========================================
  Files          41       41              
  Lines        2072     3064     +992     
==========================================
+ Hits         2045     3026     +981     
- Misses         15       26      +11     
  Partials       12       12              

Flag	Coverage Δ
go-1.13	98.75% <100.00%> (?)
go-1.14	98.59% <100.00%> (?)
go-1.15	98.59% <100.00%> (?)
go-1.16	98.59% <100.00%> (?)
macos-latest	98.75% <100.00%> (?)
nomsgpack	98.74% <100.00%> (?)
ubuntu-latest	98.75% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
routergroup.go	100.00% <100.00%> (ø)
render/json.go	84.04% <0.00%> (-1.26%)	⬇️
gin.go	99.09% <0.00%> (-0.09%)	⬇️
utils.go	96.80% <0.00%> (-0.02%)	⬇️
fs.go	100.00% <0.00%> (ø)
auth.go	100.00% <0.00%> (ø)
mode.go	100.00% <0.00%> (ø)
path.go	100.00% <0.00%> (ø)
tree.go	100.00% <0.00%> (ø)
debug.go	100.00% <0.00%> (ø)
... and 32 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 34ce210...2a24581. Read the comment docs.

[codenoid]

any update?

[appleboy]

@thinkerou Please help to review.

[thinkerou]

lgtm