pgsrv: Describe prepared statement

[justinrubek]

• Describe
  • return ParameterDescription for all parameters needed by the statement
  • return a RowDescription describing the rows that will be returned during execution

In order to provide ParameterDescription we need Parse to be able to detect placeholders #122.
The RowDescription may require some level of planning to determine.

postgres docs

The Describe message (statement variant) specifies the name of an existing prepared statement (or an empty string for the unnamed prepared statement). The response is a ParameterDescription message describing the parameters needed by the statement, followed by a RowDescription message describing the rows that will be returned when the statement is eventually executed (or a NoData message if the statement will not return rows). ErrorResponse is issued if there is no such prepared statement. Note that since Bind has not yet been issued, the formats to be used for returned columns are not yet known to the backend; the format code fields in the RowDescription message will be zeroes in this case.

[greyscaled]

I don't quite know if this is still relevant as specified. Since we have #122 , I think we can cancel this ticket?

[greyscaled]

Stale - can reopen later