[UX-749] rpk: add --format json/yaml to shadow describe

[r-vasquez]

It is pretty similar to our raw text response, the only difference is that the fields in the overview section are flattened now in the top level as it makes more sense when querying the resource.

Sample:

$ rpk shadow describe shadow-test --format json
{"name":"shadow-test","id":"f895a293-21bc-4c49-a1d4-501d3abde03e","state":"ACTIVE","client_options":{"client_id":"cluster-link-shadow-test-444-21bc-444-a1d4-444","bootstrap_servers":["444.foo.bar.cloud.redpanda.com:9092"],"tls_settings":{"enabled":true},"authentication_configuration":{"username":"foo","mechanism":"SCRAM-SHA-256","password_set":true,"password_set_at":"2025-12-19T20:13:10Z"},"metadata_max_age_ms":10000,"connection_timeout_ms":1000,"retry_backoff_ms":100,"fetch_wait_max_ms":500,"fetch_min_bytes":5242880,"fetch_max_bytes":20971520,"fetch_partition_max_bytes":5242880},"topic_metadata_sync_options":{"interval":"30s","paused":false,"start_offset":"EARLIEST","auto_create_shadow_topic_filters":[{"pattern_type":"PREFIX","filter_type":"INCLUDE","name":"shadow-"}],"synced_shadow_topic_properties":["retention.ms","compression.type","segment.ms"]},"consumer_offset_sync_options":{"interval":"30s","paused":false,"group_filters":[{"pattern_type":"LITERAL","filter_type":"INCLUDE","name":"*"}]},"security_sync_options":{"interval":"30s","paused":false,"acl_filters":[{"resource_filter":{"resource_type":"TOPIC","pattern_type":"PREFIXED","name":"test-"},"access_filter":{"principal":"User:admin","operation":"ANY","permission_type":"ALLOW","host":"*"}}]},"schema_registry_sync_options":{"shadowing_mode":"shadow schema registry topic"}}

Pretty print:

{
  "name": "shadow-test",
  "id": "f895a293-21bc-4c49-a1d4-501d3abde03e",
  "state": "ACTIVE",
  "client_options": {
    "client_id": "cluster-link-shadow-test-444-21bc-444-a1d4-444",
    "bootstrap_servers": [
      "444.foo.bar.cloud.redpanda.com:9092"
    ],
    "tls_settings": {
      "enabled": true
    },
    "authentication_configuration": {
      "username": "foo",
      "mechanism": "SCRAM-SHA-256",
      "password_set": true,
      "password_set_at": "2025-12-19T20:13:10Z"
    },
    "metadata_max_age_ms": 10000,
    "connection_timeout_ms": 1000,
    "retry_backoff_ms": 100,
    "fetch_wait_max_ms": 500,
    "fetch_min_bytes": 5242880,
    "fetch_max_bytes": 20971520,
    "fetch_partition_max_bytes": 5242880
  },
  "topic_metadata_sync_options": {
    "interval": "30s",
    "paused": false,
    "start_offset": "EARLIEST",
    "auto_create_shadow_topic_filters": [
      {
        "pattern_type": "PREFIX",
        "filter_type": "INCLUDE",
        "name": "shadow-"
      }
    ],
    "synced_shadow_topic_properties": [
      "segment.ms",
      "retention.ms",
      "compression.type"
    ]
  },
  "consumer_offset_sync_options": {
    "interval": "30s",
    "paused": false,
    "group_filters": [
      {
        "pattern_type": "LITERAL",
        "filter_type": "INCLUDE",
        "name": "*"
      }
    ]
  },
  "security_sync_options": {
    "interval": "30s",
    "paused": false,
    "acl_filters": [
      {
        "resource_filter": {
          "resource_type": "TOPIC",
          "pattern_type": "PREFIXED",
          "name": "test-"
        },
        "access_filter": {
          "principal": "User:admin",
          "operation": "ANY",
          "permission_type": "ALLOW",
          "host": "*"
        }
      }
    ]
  },
  "schema_registry_sync_options": {
    "shadowing_mode": "shadow schema registry topic"
  }
}

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

Improvements

• rpk shadow describe now supports the --format json/yaml flag.

[Copilot]

Pull request overview

This PR adds JSON and YAML output format support to the rpk shadow describe command. Previously, the command only supported text output with configurable sections. Now users can output the complete shadow link configuration in structured formats suitable for programmatic consumption or configuration management.

Key Changes:

• Added shadowLinkDescription and related structs to represent the unified output format
• Modified printShadowLinkDescription and printCloudShadowLinkDescription to support formatters
• Implemented conversion functions to transform protobuf types into the output structs
• Added the --format flag to the command

[r-vasquez]

For the record, copilot review is mistaken, probably misguided as both core and cloud share some proto definitions. Go build would have failed if this was the case.

[r-vasquez]

/backport v25.3.x