Support for retrieving metadata on current commit/t (i.e. head commit)

[aaj3f]

Description

There is a need to retrieve metadata on the current head commit, without already knowing the t value of that commit. For example, I have no idea what the current commit is, but I need to know the datetime instant of the most recent commit -- I can't use an endpoint like /commit because I don't have a t value to provide. In Fluree v2, I could directly query _block data that would have been added by the transaction server on each new block commit, e.g.

{
	selectOne: ["_block/number", "_block/instant"],
	from: "_block",
	opts: {
		orderBy: ["DESC", "_block/number"],
	},
}

but so far as I can tell, regardless of connection type and regardless of whether you are querying a ledger that has had all changes committed to it, no such commit metadata exists.

Acceptance Criteria

• Either by issuing a query directly or via some other endpoint (e.g. ledger-info, commit, etc) it should be possible to ascertain ledger metadata such as the current head commit
• If possible, it would be nice to also retrieve the desired metadata ABOUT that commit (e.g. datetime instant) without needing to issue a subsequent API call

Implementation Details

[aaj3f]

@mpoffald @dpetran I suppose could be related to #333 if there was a decision made to allow, for example, a call to /commit with no specified t value to then return the current commit at the head of the ledger