cli: live-slots now displays the rate the root slot is advancing

[mvines]

This new feature to solana live-slots nicely highlights the PoH regression in #8445

SLP:

$ solana live-slots --ws ws://34.82.79.31:8900
⠤ SlotInfoMessage { parent: 55018, root: 54987, slot: 55019 } | root slot advancing at 0.80 slots/second

Testnet on 0.23.2:

$ solana live-slots --ws ws://35.203.147.197:8900
⠙ SlotInfoMessage { parent: 30538, root: 30507, slot: 30539 } | root slot advancing at 2.00 slots/second

Fixes #8452

[codecov]

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