config: introduce pseudo "tenants" zone

Fixes cockroachdb#49318.
Fixes cockroachdb#49445.
Progress towards cockroachdb#48123.
Informs cockroachdb#48774.

This commit introduces a new pseudo object ID in the system tenant's
namespace called "tenants". Like "liveness" and "timeseries" before it,
the pseudo object allows zone configurations to be applied to
pseudo-objects that do not live directly in the system tenant's SQL
keyspace. In this case, the "tenants" zone allows zone configurations to
be set by the system tenant and applied to all other tenants in the
system. There may come a time when we want secondary tenants to have
more control over their zone configurations, but that will require a
much larger change to the zone config structure and UX as a whole.

While making this change, we rationalize the rest of zone configuration
handling and how it relates to multi-tenancy. Now that secondary tenant
ranges have a zone config to call their own, we can make sense of calls
from KV into the zone configuration infrastructure. We gate off calls
that don't make sense for secondary tenants and clean up hooks in SQL
that handle zone config manipulation.

All of this works towards a good cause - we eliminate the remaining uses
of `keys.TODOSQLCodec` from `pkg/sql/...` and `pkg/config/...`, bringing
us a big step closer towards being able to remove the placeholder and
close cockroachdb#48123.

This work also reveals that in order to address cockroachdb#48774, we need to be
able to determine split points from the SystemConfig. This makes it very
difficult to split on secondary tenant object (e.g. table) boundaries.
However, it makes it straightforward to split on secondary tenant
keysapce boundaries. This is already what we were thinking (see cockroachdb#47907),
so out both convenience and desire, I expect that we'll follow this up
with a PR that splits Ranges only at secondary tenant boundaries -
placing the overhead of an otherwise empty tenant at only a single Range
and a few KBs of data.

pkg/ccl/backupccl/backup_test.go

@@ -1228,7 +1228,7 @@ func TestBackupRestoreControlJob(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		last := uint32(v.ValueInt())
+		last := config.SystemTenantObjectID(v.ValueInt())
 		zoneConfig := zonepb.DefaultZoneConfig()
 		zoneConfig.RangeMaxBytes = proto.Int64(5000)
 		config.TestingSetZoneConfig(last+1, zoneConfig)

pkg/ccl/importccl/exportcsv_test.go

@@ -55,7 +55,7 @@ func setupExportableBank(t *testing.T, nodes, rows int) (*sqlutils.SQLRunner, st
 	if err != nil {
 		t.Fatal(err)
 	}
-	last := uint32(v.ValueInt())
+	last := config.SystemTenantObjectID(v.ValueInt())
 	zoneConfig := zonepb.DefaultZoneConfig()
 	zoneConfig.RangeMaxBytes = proto.Int64(5000)
 	config.TestingSetZoneConfig(last+1, zoneConfig)

pkg/ccl/partitionccl/partition_test.go

@@ -1406,7 +1406,7 @@ ALTER TABLE t ALTER PRIMARY KEY USING COLUMNS (y)
 
 	// Get the zone config corresponding to the table.
 	table := sqlbase.GetTableDescriptor(kvDB, keys.SystemSQLCodec, "t", "t")
-	kv, err := kvDB.Get(ctx, config.MakeZoneKey(uint32(table.ID)))
+	kv, err := kvDB.Get(ctx, config.MakeZoneKey(config.SystemTenantObjectID(table.ID)))
 	if err != nil {
 		t.Fatal(err)
 	}

pkg/config/keys.go

@@ -17,19 +17,20 @@ import (
 
 // MakeZoneKeyPrefix returns the key prefix for id's row in the system.zones
 // table.
-func MakeZoneKeyPrefix(id uint32) roachpb.Key {
-	return keys.SystemSQLCodec.ZoneKeyPrefix(id)
+func MakeZoneKeyPrefix(id SystemTenantObjectID) roachpb.Key {
+	return keys.SystemSQLCodec.ZoneKeyPrefix(uint32(id))
 }
 
 // MakeZoneKey returns the key for id's entry in the system.zones table.
-func MakeZoneKey(id uint32) roachpb.Key {
-	return keys.SystemSQLCodec.ZoneKey(id)
+func MakeZoneKey(id SystemTenantObjectID) roachpb.Key {
+	return keys.SystemSQLCodec.ZoneKey(uint32(id))
 }
 
-// DecodeObjectID decodes the object ID from the front of key. It returns the
-// decoded object ID, the remainder of the key, and whether the result is valid
-// (i.e., whether the key was within the structured key space).
-func DecodeObjectID(key roachpb.RKey) (uint32, []byte, bool) {
-	rem, id, err := keys.TODOSQLCodec.DecodeTablePrefix(key.AsRawKey())
-	return id, rem, err == nil
+// DecodeSystemTenantObjectID decodes the object ID for the system-tenant from
+// the front of key. It returns the decoded object ID, the remainder of the key,
+// and whether the result is valid (i.e., whether the key was within the system
+// tenant's structured key space).
+func DecodeSystemTenantObjectID(key roachpb.RKey) (SystemTenantObjectID, []byte, bool) {
+	rem, id, err := keys.SystemSQLCodec.DecodeTablePrefix(key.AsRawKey())
+	return SystemTenantObjectID(id), rem, err == nil
 }

pkg/config/keys_test.go

@@ -21,14 +21,14 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
 )
 
-func TestDecodeObjectID(t *testing.T) {
+func TestDecodeSystemTenantObjectID(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 
 	testCases := []struct {
 		key       roachpb.RKey
 		keySuffix []byte
 		success   bool
-		id        uint32
+		id        config.SystemTenantObjectID
 	}{
 		// Before the structured span.
 		{roachpb.RKeyMin, nil, false, 0},
@@ -43,7 +43,7 @@ func TestDecodeObjectID(t *testing.T) {
 	}
 
 	for tcNum, tc := range testCases {
-		id, keySuffix, success := config.DecodeObjectID(tc.key)
+		id, keySuffix, success := config.DecodeSystemTenantObjectID(tc.key)
 		if success != tc.success {
 			t.Errorf("#%d: expected success=%t", tcNum, tc.success)
 			continue