sql: make it easier to get full statements from node_distsql_flows and cluster_distsql_flows

Two virtual tables, `crdb_internal.node_distsql_flows` and `crdb_internal.cluster_distsql_flows`, are useful for finding hot or long-running DistSQL flows on remote nodes. But it can be difficult to get the full statement from these tables, which include only the first 500 characters and no placeholder values. Here are two ideas for enhancements:
1. Include the full statement (with placeholder values) in these virtual tables.
2. Include additional columns to make it easier to join to other virtual tables (e.g. session_id, txn_id, gateway_node_id, etc).

Jira issue: CRDB-24692