Add proposer call time to historical dataset

**Is your feature request related to a problem? Please describe.**
There has been a lot of interest around timing games recently and one of the core metrics for this is when the proposer calls the auction. Right now the best approximate we have is to match the winning bid to its submission time but this is a lower bound (see https://github.com/dataalways/mevboost-data/tree/main).

**Describe the solution you'd like**
Add the time that the proposer requested the block header during the slot to the historical data API

**Additional context**

https://ethresear.ch/t/timing-games-implications-and-possible-mitigations/17612/2

https://arxiv.org/abs/2305.09032