Add two new commands to dump and summarize a block index

[mdisibio]

What this PR does:
This PR adds two new commands that may be helpful for analysis:

list index Displays count/smallest/largest index entries for the block
Example:

$ ./tempo-cli -c ~/tempo.gcs.yaml list index 1 eb9097da-ce9b-4c79-a95e-2544e47eb600
Index entries: 85565
Min record: ID:000000000000000000f98dff14b6d9d8 Start:376637542 Length:88266
Max record: ID:00000000000000003ac4feb1e33cb8be Start:8797836709 Length:16873958

view index Dumps index entries to stdout with pauses for enter key
Example:

$ ./tempo-cli -c ~/tempo.gcs.yaml view index 1 8007402d-e2bf-4867-885b-56667d1a21bb
.
.
.
Index entry:      27694     ID: 0000000000000000091531809b380fca     Start: 3681618906     Length:     131629
Index entry:      27695     ID: 000000000000000009154701495d4bcc     Start: 3681750535     Length:     143781
Index entry:      27696     ID: 0000000000000000091557765faccdba     Start: 3681894316     Length:     135249
Index entry:      27697     ID: 000000000000000009156dc402f1a74a     Start: 3682029565     Length:     130927
Index entry:      27698     ID: 00000000000000000915821eebbb210c     Start: 3682160492     Length:     138300
Index entry:      27699     ID: 0000000000000000091594c70025a539     Start: 3682298792     Length:     132232
Press enter to continue

Which issue(s) this PR fixes:
n/a

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[annanay25]

I like the command naming and functionality. Also as for exposing a function from the encoding package, I think we should be OK as long as we're using that just for the CLI and not in other packages.