print of `teal_data` should include `datanames` (and also possibly `join_keys`)

[pawelru]

Feature description

teal might throw that data object (of teal_data class) is missing datanames attribute however this is not very straightforward to check its content. One needs to know teal.data package API to get to know this. I think we can make it simpler and print this when printing the whole object so that the knowledge of package API is no longer needed.

Currently, only validated attribute is included.

Function to modify: https://github.com/insightsengineering/teal.data/blob/870e7a4b7594ffa2124dc3954cbca47869028a42/R/teal_data-show.R#L13-L20

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