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

1 files changed, 29 insertions, 0 deletions

diff --git a/weed/security/jwt.go b/weed/security/jwt.go
index 1976c8ffe..82ba0df12 100644
--- a/weed/security/jwt.go
+++ b/weed/security/jwt.go
@@ -20,6 +20,13 @@ type SeaweedFileIdClaims struct {
 	jwt.StandardClaims
 }
 
+// SeaweedFilerClaims is created e.g. by S3 proxy server and consumed by Filer server.
+// Right now, it only contains the standard claims; but this might be extended later
+// for more fine-grained permissions.
+type SeaweedFilerClaims struct {
+	jwt.StandardClaims
+}
+
 func GenJwtForVolumeServer(signingKey SigningKey, expiresAfterSec int, fileId string) EncodedJwt {
 	if len(signingKey) == 0 {
 		return ""
@@ -41,6 +48,28 @@ func GenJwtForVolumeServer(signingKey SigningKey, expiresAfterSec int, fileId st
 	return EncodedJwt(encoded)
 }
 
+// GenJwtForFilerServer creates a JSON-web-token for using the authenticated Filer API. Used f.e. inside
+// the S3 API
+func GenJwtForFilerServer(signingKey SigningKey, expiresAfterSec int) EncodedJwt {
+	if len(signingKey) == 0 {
+		return ""
+	}
+
+	claims := SeaweedFilerClaims{
+		jwt.StandardClaims{},
+	}
+	if expiresAfterSec > 0 {
+		claims.ExpiresAt = time.Now().Add(time.Second * time.Duration(expiresAfterSec)).Unix()
+	}
+	t := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	encoded, e := t.SignedString([]byte(signingKey))
+	if e != nil {
+		glog.V(0).Infof("Failed to sign claims %+v: %v", t.Claims, e)
+		return ""
+	}
+	return EncodedJwt(encoded)
+}
+
 func GetJwt(r *http.Request) EncodedJwt {
 
 	// Get token from query params