GraphQL: deprecate `StorageMapping.spec`, add structured `fragmentStores` and `dataPlanes` fields

[GregorShear]

StorageMapping exposes its storage definition as an untyped spec: JSON! field, so clients must hand-type the blob.

Desired API contract:

• spec marked @deprecated (removed once the UI adopts the new fields)
• Structured output fields for the spec's contents:

query {
    storageMappings(filter: { catalogPrefix: { startsWith: "acmeCo/" } }) {
        edges {
            node {
                catalogPrefix
                spec                    # deprecated in favor of dataPlanes and fragmentStores
                dataPlanes {
                    name
                    cloudProvider
                    region
                    isPublic
                }
                fragmentStores {
                    provider
                    prefix
                    bucket              # null for Azure stores
                    region              # null for GCS/Azure stores
                    storageAccountName  # null for S3/GCS/Custom stores
                    containerName       # null for S3/GCS/Custom stores
                }
            }
        }
    }
}

Note: check user's authz access to each dataPlane.dataPlaneName and simply omit any that the user lacks access to