net/http: add FS to convert fs.FS to FileSystem

net/http: add FS to convert fs.FS to FileSystem

Two different functions in the http API expect a FileSystem:
http.FileSystem and http.NewFileTransport.
Add a general converter http.FS to turn an fs.FS into an http.FileSystem
for use with either of these functions.

(The original plan was to add http.HandlerFS taking an fs.FS directly,
but that doesn't help with NewFileTransport.)

For #41190.

Change-Id: I5f242eafe9b963f4387419a2615bdb487c358f16
Reviewed-on: https://go-review.googlesource.com/c/go/+/243939
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>