sql: investigate whether mandatory splits at table and index boundaries should be used

**Describe the problem**

During both index creation and import processing we opt out of splitting at table boundaries for non-system tenant:

```
pkg/sql/index_split_scatter.go:55:15: if !is.codec.ForSystemTenant() || indexToBackfill.IsTemporaryIndexForBackfill() {
```

```
pkg/sql/importer/import_processor_planning.go:217:19: if evalCtx.Codec.ForSystemTenant() {
```

```
pkg/sql/backfill.go:2242:22: if sc.execCfg.Codec.ForSystemTenant() {
```

The ingestion processor will still do splits for performance, so the exact impact of not doing these initial set of splits is unclear to me. Since secondary tenants can issue splits if the have the CanAdminSplit capability, we should consider whether we should try these splits unconditionally.

Jira issue: CRDB-34661