refactor(bigquery/storage/managedwriter): improve readability (#5239)

This PR introduces no functional changes.  It simply colocates the
two types of variadic options into the same file to aid readability.

Towards: #4366

bigquery/storage/managedwriter/appendresult.go

@@ -20,7 +20,6 @@ import (
 	storagepb "google.golang.org/genproto/googleapis/cloud/bigquery/storage/v1"
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/types/descriptorpb"
-	"google.golang.org/protobuf/types/known/wrapperspb"
 )
 
 // NoStreamOffset is a sentinel value for signalling we're not tracking
@@ -114,23 +113,3 @@ func (pw *pendingWrite) markDone(startOffset int64, err error, fc *flowControlle
 		fc.release(pw.reqSize)
 	}
 }
-
-// AppendOption are options that can be passed when appending data with a managed stream instance.
-type AppendOption func(*pendingWrite)
-
-// UpdateSchemaDescriptor is used to update the descriptor message schema associated
-// with a given stream.
-func UpdateSchemaDescriptor(schema *descriptorpb.DescriptorProto) AppendOption {
-	return func(pw *pendingWrite) {
-		pw.newSchema = schema
-	}
-}
-
-// WithOffset sets an explicit offset value for this append request.
-func WithOffset(offset int64) AppendOption {
-	return func(pw *pendingWrite) {
-		pw.request.Offset = &wrapperspb.Int64Value{
-			Value: offset,
-		}
-	}
-}

bigquery/storage/managedwriter/writer_option.go → bigquery/storage/managedwriter/options.go

@@ -17,6 +17,7 @@ package managedwriter
 import (
 	"github.com/googleapis/gax-go/v2"
 	"google.golang.org/protobuf/types/descriptorpb"
+	"google.golang.org/protobuf/types/known/wrapperspb"
 )
 
 // WriterOption are variadic options used to configure a ManagedStream instance.
@@ -96,3 +97,23 @@ func WithAppendRowsCallOption(o gax.CallOption) WriterOption {
 		ms.callOptions = append(ms.callOptions, o)
 	}
 }
+
+// AppendOption are options that can be passed when appending data with a managed stream instance.
+type AppendOption func(*pendingWrite)
+
+// UpdateSchemaDescriptor is used to update the descriptor message schema associated
+// with a given stream.
+func UpdateSchemaDescriptor(schema *descriptorpb.DescriptorProto) AppendOption {
+	return func(pw *pendingWrite) {
+		pw.newSchema = schema
+	}
+}
+
+// WithOffset sets an explicit offset value for this append request.
+func WithOffset(offset int64) AppendOption {
+	return func(pw *pendingWrite) {
+		pw.request.Offset = &wrapperspb.Int64Value{
+			Value: offset,
+		}
+	}
+}