Support casting primary keys

[nehanene15]

There should be support to cast a primary key block in the YAML like so:

primary_keys:
  - cast: date
    field_alias: start_time
    source_column: start_time
    target_column: start_time

This issue is particularly focusing on a Teradata to BQ validaton

[srikanthgaddam2]

@nehanene15 We need support for casting as Datetime

[nehanene15]

@nehanene15 We need support for casting as Datetime

What is the original data type of the field you want to cast to DATETIME? What is the query you are expecting on the TD side and on the BQ side?

For example, is it something like this?
Teradata: CAST(timestamp_col AS DATE) compared to
BigQuery: CAST(timestamp_col AS DATETIME)

[srikanthgaddam2]

The original datatype on teradata and big query is Timestamp with timezone on teradata and Timestamp on Big query