table_transformers support for "template header" or "template variables"

[thorfi]

Is your feature request related to a problem? Please describe.

We are generating many template column transformers, in a number of these we re-use data constants or block code. It would be helpful to have shared header blocks that get applied in template blocks.

Describe the solution you'd like

Something like this:

transformations:
  table_transformer_template_header: |-
    {{- $global_sequence_one := (list 1 2 3 4 5 4 3 2 1) -}}
    {{- $global_sequence_two := (list 6 4 2 0) -}}

  table_transformers:
    - schema: public
      table: logs
      column_transfomer_template_header: |-
        {{- $table_sequence_one := (list "a" "b" "c" "x" "y" "z") -}}
        {{- $table_sequence_two  := (list "q" "r" "s") -}}
      column_transformers:
        log_message:
          name: template
          parameters:
            template:
              {{- index $table_sequence_one (index $global_sequence_one 2) -}}

Meaning that the effective template code would be:

      column_transformers:
        log_message:
          name: template
          parameters:
            template: |-
              {{- $global_sequence_one := (list 1 2 3 4 5 4 3 2 1) -}}
              {{- $global_sequence_two := (list 6 4 2 0) -}}
              {{- $table_sequence_one := (list "a" "b" "c" "x" "y" "z") -}}
              {{- $table_sequence_two  := (list "q" "r" "s") -}}
              {{- index $table_sequence_one (index $global_sequence_one 2) -}}

Describe alternatives you've considered

Currently I just generate template code and dump duplicate blocks everywhere in the config file...

Alternatively (or additionally), a config section that sets variables would be good

transformations:
  table_transformer_template_variables:
    global_sequence_one: [ 1, 2, 3, 4, 5, 4, 3, 2, 1 ]
    global_sequence_two:
       - 6
       - 4
       - 2
       - 0
  table_transformers:
    - schema: public
      table: logs
      column_transfomer_template_variables:
        table_sequence_one: [ "a", "b", "c", "x", "y", "z"]
        table_sequence_two:
          - "q"
          - "r"
          - "s"
      column_transformers:
        log_message:
          name: template
          parameters:
            template:
              {{- index $table_sequence_one (index $global_sequence_one 2) -}}