go.mod: add tool section in module file (#32598)

This removes the tools.go workaround in favor of the official
tool management infrastructure, which was added in Go 1.24.

Author: markya0616

build/tools/tools.go

@@ -26,7 +26,6 @@ require (
 	github.com/ethereum/go-verkle v0.2.2
 	github.com/fatih/color v1.16.0
 	github.com/ferranbt/fastssz v0.1.4
-	github.com/fjl/gencodec v0.1.0
 	github.com/fsnotify/fsnotify v1.6.0
 	github.com/gballet/go-libpcsclite v0.0.0-20190607065134-2772fd86a8ff
 	github.com/gofrs/flock v0.12.1
@@ -103,6 +102,7 @@ require (
 	github.com/deepmap/oapi-codegen v1.6.0 // indirect
 	github.com/dlclark/regexp2 v1.7.0 // indirect
 	github.com/emicklei/dot v1.6.2 // indirect
+	github.com/fjl/gencodec v0.1.0 // indirect
 	github.com/garslo/gogen v0.0.0-20170306192744-1d203ffc1f61 // indirect
 	github.com/getsentry/sentry-go v0.27.0 // indirect
 	github.com/go-ole/go-ole v1.3.0 // indirect
@@ -146,3 +146,9 @@ require (
 	golang.org/x/net v0.38.0 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 )
+
+tool (
+	github.com/fjl/gencodec
+	golang.org/x/tools/cmd/stringer
+	google.golang.org/protobuf/cmd/protoc-gen-go
+)