should close https://github.com/nushell/nushell/issues/9774
# Description
given the help page of `into datetime`,
```
Parameters:
...rest <cellpath>: for a data structure input, convert data at the given cell paths
```
it looks like `into datetime` should accept tables as input 🤔
this PR
- adds the `table -> table` signature to `into datetime`
- adds a test to make sure the behaviour stays there
