Extend slot start message and default status bar with information about
current head fork and the next fork transition (corresponding to head).
This is useful to know whether a synced client is aware of a future fork
and can also be useful when syncing from old forks to follow progress
across the various forks.
```
peers: 8 ❯ finalized: 741c2ce2:230474 ❯ head: b330f58b:230477:20 ❯ fork: Capella (next: Deneb:231680) ❯ time: 230599:24 (7379192) ❯ sync: 00h24m (99.63%) 2.6492slots/s (QwQUwQPQDQ:7375263)/opt
```
```
INF 2024-01-12 12:18:00.001+01:00 Slot start topics="beacnde" slot=7379190 epoch=230599 fork="Capella (next: Deneb:231680)" sync="--h--m (99.62%) 0.0000slots/s (wwwwwwwwww:7375167)/opt" peers=0 head=741c2ce2:7375168 finalized=230472:723abe7e delay=1ms861us
```
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