Feat: Allow for embedding of decK configuration within Control Plane resource config

As of now, users must have deck file declarations in an independent file which the `_deck` declaration references:

```
control_planes:
- ref: example-cp
name: "example-cp"
description: "Example Control Plane managed by kongctl"
cluster_type: "CLUSTER_TYPE_SERVERLESS"
_deck:
files:
- "gateway-services.yaml"
flags:
- "--analytics=false"

gateway_services:
- ref: example-gw-svc
_external:
selector:
matchFields:
name: "example-http-service"
```

The request is to allow for the deck configuration to be embedded directly instead of requiring the external file. Something like this:

```
control_planes:
- ref: example-cp
name: "example-cp"
description: "Example Control Plane managed by kongctl"
cluster_type: "CLUSTER_TYPE_SERVERLESS"
_deck:
config:
_format_version: "3.0"
_info:
select_tags:
- "kongctl-example"
services:
- name: example-http-service
url: http://httpbin.konghq.com
tags:
- kongctl-example
routes:
- name: example-http-route
paths:
- /example
protocols:
- http
- https
tags:
- kongctl-example
flags:
- "--analytics=false"
gateway_services:
- ref: example-gw-svc
_external:
selector:
matchFields:
name: "example-http-service"
```

The above example then results in some redundant definitions, the gateway service specifically. We should evaluate ways to reduce that boilerplate if possible.

Start by tracing how the existing `_deck.files` declarations are read and how the embedded control-plane configuration is parsed. Compare the external-file and proposed inline forms, including the redundant gateway service definition, and document a clear approach that supports inline configuration while addressing the boilerplate concern.

Written by the indexing model from the issue text.