Commit attributes

[snazy]

Out of a recent off-line discussion, we might want to have more attributes in a commit as technical information.

Currently we have the commit message and a bunch of attributes like committer, commit timestamp, author and author timestamp. These are analogous to Git.

With Nessie however, we might want additional attributes, that we need to "standardize" on:

• technical, immutable user ID (if available)
• identifier of the engine/client that performs the commit (if available)

The "new" (current) storage model has a rather flexible way to add arbitrary headers to each commit (o.p.versioned.storage.common.objtypes.CommitHeaders). The public facing API however is rather strict (o.p.model.CommitMeta).