flist: support metadata (tags)

[muhamadazmy]

Currently, we have to maintain the version in flist name. This works fine for most of the cases, but in some usecases, we always want a known fixed flist (no version in the name) but we still need to know what version this flist is. Hence we need to maintain a tag set that can be set during the upload of the flist.

Suggestion:
I think this can be easily implemented by using the xattr on the flist files (using the attr command)

[maxux]

Or we just use flist metadata ?

[maxux]

This is now supported on playground: https://playground.hub.grid.tf/api/flist/maxux42.3bot/demo-metadata.flist/metadata
We can set metadata via upload API, not via webui yet. This will come later.