feat: Add flag to render allowing DECK_ env var processing

[rspurgeon]

Resolves #1207

By default deck file render mocks the values of DECK_ env vars. This was intentional as to support rendering a full configuration in environments where all values are not present (ex: CI systems). This PR adds a flag for deck file render, --populate-env-vars with a default of false. The flag is written in the affirmative case while the false default prevents breaking previous default behavior. Users interested in populating the env vars must set --populate-env-vars to enable the behavior.

DECK_ECHO_DOWNSTREAM=true DECK_HEADER_NAME=X-Flag ./deck file render ~/kong/tmp/deck-file-render-1207.yaml
_format_version: "3.0"
plugins:
- config:
    echo_downstream: false
    generator: uuid
    header_name: DECK_HEADER_NAME
  enabled: true
  name: correlation-id
  protocols:
  - grpc
  - grpcs
  - http
  - https

> DECK_ECHO_DOWNSTREAM=true DECK_HEADER_NAME=X-Flag ./deck file render ~/kong/tmp/deck-file-render-1207.yaml --populate-env-vars
_format_version: "3.0"
plugins:
- config:
    echo_downstream: true
    generator: uuid
    header_name: X-Flag
  enabled: true
  name: correlation-id
  protocols:
  - grpc
  - grpcs
  - http
  - https

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (9efd8b4) 5.53% compared to head (8abae6e) 5.53%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
cmd/file_render.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1208      +/-   ##
========================================
- Coverage   5.53%   5.53%   -0.01%     
========================================
  Files         32      32              
  Lines       3360    3363       +3     
========================================
  Hits         186     186              
- Misses      3164    3167       +3     
  Partials      10      10              

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