* fix(filer.meta.tail): include extended metadata in Elasticsearch docs
The -es sink flattened only the FUSE attributes, so xattrs (including S3
user metadata like X-Amz-Meta-*) never reached Elasticsearch. Add an
Extended field and convert map[string][]byte to map[string]string so the
values index as text; non-UTF-8 values fall back to base64.
Addresses #9190 follow-up.
* fix(filer.meta.tail): prefix base64-encoded extended values with "base64:"
Addresses review feedback: a plain UTF-8 xattr and a base64 fallback are otherwise indistinguishable to a consumer reading the ES doc.
Chris Lu