ldr: add crdb_replication.dlq_to_record builtin

Add the ability to decode DLQ events as datums. Does not necessarily need to be a builtin, could just be a helper that future builtins use. See `jsonb_to_recordset` for example.

Epic CRDB-65612

Jira issue: CRDB-65685

Start by reading the jsonb_to_recordset implementation and tracing the ldr replication code that handles DLQ events. Identify the event shape and how it should become datums, then determine whether the logic belongs in a builtin or reusable helper. Done means DLQ events decode correctly and the relevant coverage verifies the resulting records.

Written by the indexing model from the issue text.