Feature Request: Add a render or preview command to show template output with specific config values

Currently, there is no simple way to preview the fully rendered output of a Redpanda Connect template with a given set of config values. The existing workflows (rpk connect echo or rpk connect lint) either do not process templates or require writing test cases for every scenario.

Reference:
Slack thread: https://redpandacommunity.slack.com/archives/C075M18MAEA/p1749125364990089

Use Case:
As a developer, I want to quickly see what a template will render to with a specific config, for debugging and development purposes. This is especially useful when iterating on template logic or troubleshooting config issues, without having to write and maintain a test for every possible config variation.

command example:
rpk connect template render --template ./template.yaml --config ./config.yaml

template.yaml

name: kafka_input
type: input

fields:
  - name: broker_address
    type: string
    kind: list
  - name: topics
    type: string
    kind: list
  - name: cert_path
    type: string
  - name: consumer_group
    type: string

mapping: |
  root.input = { 
    "kafka": {
      "addresses" : this.broker_address,
      "topics": this.topics,
      "start_from_oldest": true,
      "consumer_group": this.consumer_group,
      "tls": {
        "enabled": true,
        "root_cas_file": "${this.cert_path}/ca.crt",
        "client_certs": [
          {
            "cert_file": "${this.cert_path}/user.crt",
            "key_file": "${this.cert_path}/user.key"
          }
        ]
      }
    }
  }

tests:
  - name: kafka_test
    config: 
      broker_address:
        - broker-1 
      topics:
        - topic-1
      cert_path: kafka-secrets
      consumer_group: app4134-composition-test
    expected:
      input:
        kafka:
          addresses:
            - broker-1 
          topics:
            - topic-1
          consumer_group: app4134-composition-test
          start_from_oldest: true
          tls:
            enabled: true
            root_cas_file: kafka-secrets/ca.crt
            client_certs:
              - cert_file: kafka-secrets/user.crt
                key_file: kafka-secrets/user.key

config.yaml

input:
  kafka_input:
    broker_address:
      - broker-1
    topics:
      - topic-1
    cert_path: demo-kafka-secret
    consumer_group: benthos-local-test-4

pipeline:
  processors:
    - mapping: |
        root.id = uuid_v4()
        root.foo = this.inner.foo
        root.body = this.outer`

expected output:

input:
  kafka:
    addresses:
      - broker-1
    topics:
      - topic-1
    start_from_oldest: true
    consumer_group: benthos-local-test-4
    tls:
      enabled: true
      root_cas_file: demo-kafka-secret/ca.crt
      client_certs:
        - cert_file: demo-kafka-secret/user.crt
          key_file: demo-kafka-secret/user.key

pipeline:
  processors:
    - mapping: |
        root.id = uuid_v4()
        root.foo = this.inner.foo
        root.body = this.outer

output:
  stdout: {}`

Benefit:
This would greatly improve the developer experience for anyone building or debugging templates, and would make Redpanda Connect templating more approachable and productive.

Start by comparing the existing rpk connect echo and rpk connect lint commands with the proposed rpk connect template render entry point. Trace how template.yaml and config.yaml should be loaded and how the fully rendered configuration should be emitted; done means the example produces the expected output without requiring a test case.