filer.sync: sign proxied chunk I/O from the per-side security file

The -a.security / -b.security files were used for gRPC TLS and the
HTTPS client but not for jwt.filer_signing, so filer-proxied chunk
reads and writes carried a token signed with the process-wide key and
failed authorization whenever the two clusters' keys differ.

LoadFilerJwtFromFile returns a FilerJwtProvider for each side's file,
which FilerSource and FilerSink now accept for proxied chunk reads and
writes. With no keys in the file or no flag, both fall back to the
process-wide jwt.filer_signing configuration as before.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/replication/source/filer_source.go

@@ -34,6 +34,7 @@ type FilerSource struct {
 	dataCenter     string
 	signature      int32
 	httpClient     *util_http_client.HTTPClient
+	jwtForFiler    security.FilerJwtProvider
 }
 
 func (fs *FilerSource) Initialize(configuration util.Configuration, prefix string) error {
@@ -67,6 +68,10 @@ func (fs *FilerSource) SetHttpClient(client *util_http_client.HTTPClient) {
 	fs.httpClient = client
 }
 
+func (fs *FilerSource) SetFilerJwtProvider(provider security.FilerJwtProvider) {
+	fs.jwtForFiler = provider
+}
+
 func (fs *FilerSource) LookupFileId(ctx context.Context, part string) (fileUrls []string, err error) {
 
 	vid2Locations := make(map[string]*filer_pb.Locations)
@@ -126,7 +131,11 @@ func (fs *FilerSource) ReadPart(fileId string, offset int64) (filename string, h
 
 	if fs.proxyByFiler {
 		fileUrl := util_http.ProxyChunkUrl(fs.address, fileId)
-		filename, header, resp, err = downloadFn(fileUrl, util_http.JwtForFilerServer(false), offset)
+		jwt := util_http.JwtForFilerServer(false)
+		if fs.jwtForFiler != nil {
+			jwt = string(fs.jwtForFiler(false))
+		}
+		filename, header, resp, err = downloadFn(fileUrl, jwt, offset)
 		if err == nil {
 			err = readPartStatusError(fileUrl, resp)
 		}