feat: add deck file format command for deck/dbless conversion

[mheap]

Why

Ref: https://github.com/Kong/go-apiops/blob/main/deckformat/fileformat.md

Kong configuration files exist in two incompatible formats — decK (human-friendly, intended for manual editing) and DBless (Kong's backup/restore format, used with kong config db_export / kong config db_import). Users working across both toolchains need a way to convert between them without manual editing.

What

Adds a new deck file format subcommand:

deck file format deck /path/to/dbless.yaml    # DBless → decK
deck file format dbless /path/to/deck.yaml    # decK → DBless

Supports -o / --output-file (default: stdout) and --format yaml|json.

How

The two formats differ primarily in how consumer-group entities are represented:

Entity	decK	DBless
Group plugins	nested under consumer_groups[*].plugins	top-level consumer_group_plugins array
Group memberships	nested under consumers[*].groups	top-level consumer_group_consumers array

• DBless → decK delegates to deckformat.ConvertDBless() from go-apiops, which already implements this transformation.
• decK → DBless is implemented locally in convertDeckToDBless() as the inverse: it extracts nested plugins and group memberships back into top-level arrays, adding the appropriate consumer_group / consumer foreign-key fields.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 12.61261% with 97 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.26%. Comparing base (6719b2a) to head (0d15c26).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/file_format.go	0.00%	96 Missing ⚠️
cmd/root.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1978      +/-   ##
==========================================
- Coverage   32.58%   32.26%   -0.32%     
==========================================
  Files          77       78       +1     
  Lines        6903     7014     +111     
==========================================
+ Hits         2249     2263      +14     
- Misses       4474     4571      +97     
  Partials      180      180              

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