Add summarization for view column

[moritzbeck13]

Is your feature request related to a problem? Please describe.

I want to quickly see e.g. the sum of all entries or the total number of entries for a column in a view.

Describe the solution you'd like

A header field which allows me to specify different types of summarization for each column individually.

Describe alternatives you've considered

Manually counting.

Additional context

No response

[enjeck]

Related: https://github.com/nextcloud/tables/issues/36