implement count built-ins for DaphneDSL

sth. to count a specific values like `count(matrix[,0], value)` (or from a frame)(also on strings)

and/or

unique_counts as know from [numpy](https://numpy.org/doc/stable/reference/generated/numpy.unique.html)

There are currently ways to circumvent this by reshaping a matrix into single col, convert it to a frame, register it as a view and finally conduct SQL ops

Or is it the idea to provide such functionality trough SQL?

Start by locating the DaphneDSL built-in-function definitions and checking whether count operations are already exposed through SQL. Compare the requested count(matrix[,0], value), frame and string counting, and numpy.unique-style unique_counts semantics. Done means the supported scope and behavior are decided and covered by appropriate tests.

Written by the indexing model from the issue text.