sql/hints: change StatementHint protobuf struct to union

This commit makes a few changes to the protobuf message used for
serializing statement hints in the `system.statement_hints` table to
prepare for future commits/PRs. No release note or migrations are
necessary, since nothing is currently store nor read from the table.

Epic: None

Release note: None

Author: DrewKimball

pkg/sql/hints/statement_hint.go

@@ -7,18 +7,22 @@ package hints
 
 import "github.com/cockroachdb/cockroach/pkg/util/protoutil"
 
-// NewStatementHints converts the raw bytes from system.statement_hints into a
-// StatementHints object.
-func NewStatementHints(bytes []byte) (*StatementHints, error) {
-	res := &StatementHints{}
+type StatementHint interface {
+	protoutil.Message
+}
+
+// NewStatementHint converts the raw bytes from system.statement_hints into a
+// StatementHintUnion object.
+func NewStatementHint(bytes []byte) (*StatementHintUnion, error) {
+	res := &StatementHintUnion{}
 	if err := protoutil.Unmarshal(bytes, res); err != nil {
 		return nil, err
 	}
 	return res, nil
 }
 
-// ToBytes converts the StatementHints to a raw bytes representation that can be
-// inserted into the system.statement_hints table.
-func (hints *StatementHints) ToBytes() ([]byte, error) {
-	return protoutil.Marshal(hints)
+// ToBytes converts the StatementHintUnion to a raw bytes representation that
+// can be inserted into the system.statement_hints table.
+func (hint *StatementHintUnion) ToBytes() ([]byte, error) {
+	return protoutil.Marshal(hint)
 }

pkg/sql/hints/statement_hint.proto

@@ -8,8 +8,10 @@ package hints;
 
 import "gogoproto/gogo.proto";
 
-// StatementHints consolidates the "external" hints that can be applied to
-// statements with a given fingerprint using the system.statement_hints table.
-message StatementHints {
+// StatementHintsUnion contains exactly one type of "external" statement hint
+// that can be applied to statements with a given fingerprint using the
+// system.statement_hints table.
+message StatementHintUnion {
   // TODO: add a field for each type of statement hint.
+  // oneof value {}
 }