PERF: vbench for mixed groupby with datetime (GH7555)
closes #7555

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_mixed_first                          |  10.8150 | 3900.5923 |   0.0028 |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [bc5599a] : PERF: vbench for mixed groupby with datetime (GH7555)

PERF: perform coercing and casting on groupby agg by blocks
Base   [2bd4517] : Merge pull request #7544 from sinhrks/dtmixin
```
