FEATURE: add JWT to HTTP endpoints of Filer and use them in S3 Client

- one JWT for reading and one for writing, analogous to how the JWT
  between Master and Volume Server works
- I did not implement IP `whiteList` parameter on the filer

Additionally, because http_util.DownloadFile now sets the JWT,
the `download` command should now work when `jwt.signing.read` is
configured. By looking at the code, I think this case did not work
before.

## Docs to be adjusted after a release

Page `Amazon-S3-API`:

```
# Authentication with Filer

You can use mTLS for the gRPC connection between S3-API-Proxy and the filer, as
explained in [Security-Configuration](Security-Configuration) -
controlled by the `grpc.*` configuration in `security.toml`.

Starting with version XX, it is also possible to authenticate the HTTP
operations between the S3-API-Proxy and the Filer (especially
uploading new files). This is configured by setting
`filer_jwt.signing.key` and `filer_jwt.signing.read.key` in
`security.toml`.

With both configurations (gRPC and JWT), it is possible to have Filer
and S3 communicate in fully authenticated fashion; so Filer will reject
any unauthenticated communication.
```

Page `Security Overview`:

```
The following items are not covered, yet:

- master server http REST services

Starting with version XX, the Filer HTTP REST services can be secured
with a JWT, by setting `filer_jwt.signing.key` and
`filer_jwt.signing.read.key` in `security.toml`.

...

Before version XX: "weed filer -disableHttp", disable http operations, only gRPC operations are allowed. This works with "weed mount" by FUSE. It does **not work** with the [S3 Gateway](Amazon S3 API), as this does HTTP calls to the Filer.
Starting with version XX: secured by JWT, by setting `filer_jwt.signing.key` and `filer_jwt.signing.read.key` in `security.toml`. **This now works with the [S3 Gateway](Amazon S3 API).**

...

# Securing Filer HTTP with JWT

To enable JWT-based access control for the Filer,

1. generate `security.toml` file by `weed scaffold -config=security`
2. set `filer_jwt.signing.key` to a secret string - and optionally filer_jwt.signing.read.key` as well to a secret string
3. copy the same `security.toml` file to the filers and all S3 proxies.

If `filer_jwt.signing.key` is configured: When sending upload/update/delete HTTP operations to a filer server, the request header `Authorization` should be the JWT string (`Authorization: Bearer [JwtToken]`). The operation is authorized after the filer validates the JWT with `filer_jwt.signing.key`.

If `filer_jwt.signing.read.key` is configured: When sending GET or HEAD requests to a filer server, the request header `Authorization` should be the JWT string (`Authorization: Bearer [JwtToken]`). The operation is authorized after the filer validates the JWT with `filer_jwt.signing.read.key`.

The S3 API Gateway reads the above JWT keys and sends authenticated
HTTP requests to the filer.
```

Page `Security Configuration`:

```
(update scaffold file)

...

[filer_jwt.signing]
key = "blahblahblahblah"

[filer_jwt.signing.read]
key = "blahblahblahblah"
```

Resolves: #158

weed/server/filer_server.go

@@ -71,6 +71,7 @@ type FilerServer struct {
 	option         *FilerOption
 	secret         security.SigningKey
 	filer          *filer.Filer
+	filerGuard     *security.Guard
 	grpcDialOption grpc.DialOption
 
 	// metrics read from the master
@@ -90,6 +91,15 @@ type FilerServer struct {
 
 func NewFilerServer(defaultMux, readonlyMux *http.ServeMux, option *FilerOption) (fs *FilerServer, err error) {
 
+	v := util.GetViper()
+	signingKey := v.GetString("jwt.filer_signing.key")
+	v.SetDefault("jwt.filer_signing.expires_after_seconds", 10)
+	expiresAfterSec := v.GetInt("jwt.filer_signing.expires_after_seconds")
+
+	readSigningKey := v.GetString("jwt.filer_signing.read.key")
+	v.SetDefault("jwt.filer_signing.read.expires_after_seconds", 60)
+	readExpiresAfterSec := v.GetInt("jwt.filer_signing.read.expires_after_seconds")
+
 	fs = &FilerServer{
 		option:                option,
 		grpcDialOption:        security.LoadClientTLS(util.GetViper(), "grpc.filer"),
@@ -106,13 +116,14 @@ func NewFilerServer(defaultMux, readonlyMux *http.ServeMux, option *FilerOption)
 		fs.listenersCond.Broadcast()
 	})
 	fs.filer.Cipher = option.Cipher
+	// we do not support IP whitelist right now
+	fs.filerGuard = security.NewGuard([]string{}, signingKey, expiresAfterSec, readSigningKey, readExpiresAfterSec)
 
 	fs.checkWithMaster()
 
 	go stats.LoopPushingMetric("filer", string(fs.option.Host), fs.metricsAddress, fs.metricsIntervalSec)
 	go fs.filer.KeepMasterClientConnected()
 
-	v := util.GetViper()
 	if !util.LoadConfiguration("filer", false) {
 		v.Set("leveldb2.enabled", true)
 		v.Set("leveldb2.dir", option.DefaultLevelDbDir)