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/s3api/s3api_server.go

@@ -3,6 +3,8 @@ package s3api
 import (
 	"fmt"
 	"github.com/chrislusf/seaweedfs/weed/pb"
+	"github.com/chrislusf/seaweedfs/weed/security"
+	"github.com/chrislusf/seaweedfs/weed/util"
 	"net/http"
 	"strings"
 	"time"
@@ -25,14 +27,25 @@ type S3ApiServerOption struct {
 }
 
 type S3ApiServer struct {
-	option *S3ApiServerOption
-	iam    *IdentityAccessManagement
+	option     *S3ApiServerOption
+	iam        *IdentityAccessManagement
+	filerGuard *security.Guard
 }
 
 func NewS3ApiServer(router *mux.Router, option *S3ApiServerOption) (s3ApiServer *S3ApiServer, 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")
+
 	s3ApiServer = &S3ApiServer{
-		option: option,
-		iam:    NewIdentityAccessManagement(option),
+		option:     option,
+		iam:        NewIdentityAccessManagement(option),
+		filerGuard: security.NewGuard([]string{}, signingKey, expiresAfterSec, readSigningKey, readExpiresAfterSec),
 	}
 
 	s3ApiServer.registerRouter(router)