sql: ability to export and import external statement hints

We currently backup and restore the external statement hints created by these plan management builtin functions:

- `information_schema.crdb_rewrite_inline_hints`
- `information_schema.crdb_set_session_variable_hint`
- `information_schema.crdb_pin_plan_gist`
- `information_schema.crdb_pin_plan_gram`

But we do not have a way to export or import the external statement hints to move them between clusters. We should add a way to do this so that statement hints created on testing clusters can be applied to production clusters.

Jira issue: CRDB-65866

Epic: CRDB-66266

Start by examining the existing backup and restore handling for external statement hints created by the four listed information_schema functions. Determine how an export/import path could move those hints between clusters, including from testing to production. Done means externally created hints can be exported from one cluster and imported into another.

Written by the indexing model from the issue text.