Support dynamic timezone values fed from a column in DATETRUNC

In the current implementation DATETRUNC()'s optional timezone param is a hard coded string. We can pass only one value to it. In some use cases, each record in the table need to be supplied with its own timezone, coming from a different column (part of the fact table itself or coming from a dim table via lookup join).

It would be great if we can support this.

No files, tests, or entry points are named. Start by locating the DATETRUNC implementation and tracing how its optional timezone parameter is handled. Done means the timezone can be supplied per record from a fact-table or joined dimension column rather than only as a hard-coded string.

Written by the indexing model from the issue text.