Enable column casting

[MichailParaskevopoulos]

Changes:

1. Table config now takes an argument column_casting.
2. Schema parsing replaces the target table's schema based on the provided column_casting.
3. The ODS/HDS upsert SQL-builder classes use the provided BigQuery function in the column casting when inserting the values from the landing zone table.

Column casting is currently supported for ODS (both full and incremental ingestions) and HDS (only snapshot).

Example column_casting:

column_casting:
  metrics_clicks: {function: "CAST({column} AS INTEGER)", output_type: INTEGER}

Use similar syntax for any BigQuery function. For instance:

column_casting:
  metrics_clicks: {function: "TIMESTAMP_MILLIS({column})", output_type: TIMESTAMP}

Resolves Jira issue: AF-73