Siphon: allow skipping columns

Extend the TableMappingConfig with column transform configuration (we might have different kinds of transformations later):

table_mapping:
  - table: issues
    schema: public
    stream: "users"
    column_transformations:
      - name: "api_key"
        transform: "ignore" # I'm open to suggestions about the naming

When parsing the LR event and we see the api_key column, we would ignore/skip the column.

Reasoning: this is a security feature to prevent pushing keys and secrets to the pub-sub system.