replication: use the side filer read key for manifest downloads and fall back per access level

Manifest chunk resolution still signed proxied downloads with the
process-wide read key, so a source filer requiring its own key 401'd on
manifest-bearing files. A side security file that set only one access
level also produced empty tokens for the other instead of inheriting the
process-wide key, and the side file loader ignored the WEED_ environment
overrides the filer itself honors.

ResolveChunkManifest/ResolveOneChunkManifest keep their signatures;
FilerJwt-aware variants thread the provider down to fetchWholeChunk,
which prefers it on proxy URLs. The side loader now applies the same
environment precedence and falls back to the process-wide signer per
missing access level.

weed/replication/source/filer_source.go

@@ -72,6 +72,12 @@ func (fs *FilerSource) SetFilerJwtProvider(provider security.FilerJwtProvider) {
 	fs.jwtForFiler = provider
 }
 
+// FilerJwt returns the side-specific filer API credential, or nil when the
+// process-wide jwt.filer_signing configuration applies.
+func (fs *FilerSource) FilerJwt() security.FilerJwtProvider {
+	return fs.jwtForFiler
+}
+
 func (fs *FilerSource) LookupFileId(ctx context.Context, part string) (fileUrls []string, err error) {
 
 	vid2Locations := make(map[string]*filer_pb.Locations)