feat: add fine-grained control for link signing (#3924)

* Determine whether the URL requires Sign

* Add File and Mem based KV

NOT TESTED: TokenKV Function

* Change Token KV func to common func.

Add File based KV func

* Remove KV, Remove Token

I found that the original Sign function is enough to complete the link signature, and only need to add simple configuration items to meet the requirements.

* Add IsStorageSigned func to judge if Signing is enabled in the storage settings.

It should be working now.

* Add a SIGN button to the management panel.

* Add enable_sign to the basic storage struct.

Can enable sign for every driver now.

Bug: When sign enabled, in download page, Copy link doesn't contain a sign.

(Not done yet)

* Fix a bug from commit 8f6c25f.

Response of fsread function does not contain sign.

* Optimize code and follow advices.

- Add back public/dist/README.md

- Enable sign when DownProxyUrl is enabled

- Merge needSign() to isEncrypt() in fsread.go

* simplify code

---------

Co-authored-by: Andy Hsu <i@nn.ci>

go.sum

@@ -111,8 +111,8 @@ github.com/golang/geo v0.0.0-20210211234256-740aa86cb551 h1:gtexQ/VGyN+VVFRXSFig
 github.com/golang/geo v0.0.0-20210211234256-740aa86cb551/go.mod h1:QZ0nwyI2jOfgRAoBvP+ab5aRr7c9x7lhGEJrKvBwjWI=
 github.com/golang/protobuf v1.5.0 h1:LUVKkCeviFUMKqHa4tXIIij/lbhnMbP7Fn5wKdKkRh4=
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-github.com/golang/snappy v0.0.1 h1:Qgr9rKW7uDUkrbSmQeiDsGa8SjGyCOGtuasMWwvp2P4=
-github.com/golang/snappy v0.0.1/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEWrmP2Q=
+github.com/golang/snappy v0.0.3 h1:fHPg5GQYlCeLIPB9BZqMVR5nR9A+IM5zcgeTdjMYmLA=
+github.com/golang/snappy v0.0.3/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEWrmP2Q=
 github.com/google/go-cmp v0.5.5 h1:Khx7svrCpmxxtHBq5j2mp/xVjsi8hQMfNLvJFAlrGgU=
 github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/gofuzz v1.0.0/go.mod h1:dBl0BpW6vV/+mYPU4Po3pmUjxk6FQPldtuIdl/M65Eg=