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/util/http_util.go

@@ -180,7 +180,16 @@ func GetUrlStream(url string, values url.Values, readFn func(io.Reader) error) e
 }
 
 func DownloadFile(fileUrl string, jwt string) (filename string, header http.Header, resp *http.Response, e error) {
-	response, err := client.Get(fileUrl)
+	req, err := http.NewRequest("GET", fileUrl, nil)
+	if err != nil {
+		return "", nil, nil, err
+	}
+
+	if len(jwt) > 0 {
+		req.Header.Add("Authorization", "BEARER "+jwt)
+	}
+
+	response, err := client.Do(req)
 	if err != nil {
 		return "", nil, nil, err
 	}
@@ -358,7 +367,7 @@ func readEncryptedUrl(fileUrl string, cipherKey []byte, isContentCompressed bool
 	return false, nil
 }
 
-func ReadUrlAsReaderCloser(fileUrl string, rangeHeader string) (io.ReadCloser, error) {
+func ReadUrlAsReaderCloser(fileUrl string, jwt string, rangeHeader string) (io.ReadCloser, error) {
 
 	req, err := http.NewRequest("GET", fileUrl, nil)
 	if err != nil {
@@ -370,6 +379,10 @@ func ReadUrlAsReaderCloser(fileUrl string, rangeHeader string) (io.ReadCloser, e
 		req.Header.Add("Accept-Encoding", "gzip")
 	}
 
+	if len(jwt) > 0 {
+		req.Header.Add("Authorization", "BEARER "+jwt)
+	}
+
 	r, err := client.Do(req)
 	if err != nil {
 		return nil, err