sheet: add pivot table command (wrapper for SQL)

[liquidaty]

How it works:

1. User enters ":pivot-table" command
2. User is prompted to provide SQL expressions defining:
   • the "group by" expression that defines the values that will be associated with each table row, which will be displayed in the first column of the output table
   • one or more aggregate "select" expressions that define additional columns
   • an optional "order by" expression
3. From the above inputs, a SQL expression is constructed in the same form as can be used via zsv sql. The resulting table is shown in a new buffer

The new buffer should support drill-down links as described in #294

[liquidaty]

Basic form of this in mysheet_extension.c