feat(reader) handle _transform field

[rainest]

Handle the Kong state _transform field, which indicates whether data should pass through applicable transforms when imported into Kong. As deck passes data through the admin API, it will always pass through transforms. deck rejects the state if _transform=false, as it cannot satisfy this request.

AFAIK, we need take no additional action for true, and that is the default per https://docs.konghq.com/gateway-oss/2.6.x/db-less-and-declarative-config/#the-declarative-configuration-format. Rejecting false outright instead of warning since that should indicate that the file contains transformed/encrypted content, and running it through transforms again will not result in the intended config.

Fix #510

[rainest]

Testing the positive case for Get() fails due to not having a Kong instance, but

11:32:38-0700 esenin $ dlv debug -- validate -s foo.yaml 
Type 'help' for list of commands.
(dlv) c
Process 3923781 has exited with status 0
(dlv) exit
11:33:10-0700 esenin $ dlv debug -- diff -s foo.yaml        
Type 'help' for list of commands.
(dlv) c
creating certificate 13c562a1-191c-4464-9b18-e5222b46035b
creating consumer yolo
creating consumer harry
creating upstream upstream1
creating target 198.51.100.11:80 for upstream 42156d4b-b339-457c-9868-60f7636b7e27
creating target 198.51.100.12:80 for upstream 42156d4b-b339-457c-9868-60f7636b7e27
creating service svc1
creating key-auth n90ts for consumer 6e6590a5-d25d-4f1c-b265-9ee6097e5bd4
creating sni demo3.example.com
creating target 198.51.100.13:80 for upstream 42156d4b-b339-457c-9868-60f7636b7e27
creating sni demo2.example.com
creating service svc2
creating sni demo1.example.com
creating service svc3
Warning: import/export of basic-authcredentials using decK doesn't work due to hashing of passwords in Kong.
creating basic-auth user1 for consumer 6e6590a5-d25d-4f1c-b265-9ee6097e5bd4
creating hmac-auth hmac-user for consumer 6e6590a5-d25d-4f1c-b265-9ee6097e5bd4
creating jwt-auth MKWeR0nu9OAUR9HrjpUG82Hbfz7ZXsIw for consumer 6e6590a5-d25d-4f1c-b265-9ee6097e5bd4
creating acl-group foo-group for consumer 6e6590a5-d25d-4f1c-b265-9ee6097e5bd4
creating route r3
creating route r2
creating route r1
creating plugin prometheus (global)
creating plugin rate-limiting for consumer 3488030a-387d-4865-b409-4216131a8a07
Summary:
  Created: 23
  Updated: 0
  Deleted: 0
Process 3924231 has exited with status 0
(dlv) exit
11:33:39-0700 esenin $ diff -u kong.yaml foo.yaml 
--- kong.yaml	2021-11-05 11:17:59.690516063 -0700
+++ foo.yaml	2021-11-05 11:32:38.433901529 -0700
@@ -1,3 +1,4 @@
+_transform: true
 _info:
   select_tags:
   - managed-by-deck

[codecov-commenter]

Codecov Report

Merging #520 (4dc48ff) into main (b359c83) will increase coverage by 0.20%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #520      +/-   ##
==========================================
+ Coverage   50.47%   50.67%   +0.20%     
==========================================
  Files          72       72              
  Lines        7870     7876       +6     
==========================================
+ Hits         3972     3991      +19     
+ Misses       3549     3535      -14     
- Partials      349      350       +1     

Impacted Files	Coverage Δ
file/types.go	68.47% <ø> (ø)
file/reader.go	57.35% <85.71%> (+25.09%)	⬆️

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[hbagdi]

@kikito Can you verify if the understanding of "_transform" here is indeed correct or not?
LGTM. Please get a +1 from someone in the gateway team on the assumption behind the PR.

[hbagdi]

@rainest can you squash all your commits into a single commit? We are good to merge after that.

[rainest]

@hbagdi auto-merge is now set to squash mode and I've removed irrelevant commit headers from the body. It'll merge once you approve.

[mflendrich]

LGTM but disabled auto-merge so as to prevent the discussion on comments from being missed.