Accept OAuth2 POST form credentials on /v2/token

containerd/buildkit fetch registry tokens with a password or refresh_token
grant form; read the credential from it so buildx pushes get a real token.

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/cavaliergopher/rpm v1.3.0
 	github.com/charmbracelet/bubbletea v1.3.10
 	github.com/charmbracelet/lipgloss v1.1.0
+	github.com/containerd/containerd/v2 v2.2.1
 	github.com/coreos/go-oidc/v3 v3.21.0
 	github.com/go-chi/chi/v5 v5.3.0
 	github.com/go-jose/go-jose/v4 v4.1.4
@@ -14,6 +15,8 @@ require (
 	github.com/jackc/pgx/v5 v5.10.0
 	github.com/klauspost/compress v1.19.2
 	github.com/minio/minio-go/v7 v7.2.0
+	github.com/opencontainers/go-digest v1.0.0
+	github.com/opencontainers/image-spec v1.1.1
 	github.com/redis/go-redis/v9 v9.20.0
 	github.com/testcontainers/testcontainers-go v0.44.0
 	github.com/testcontainers/testcontainers-go/modules/postgres v0.44.0
@@ -42,7 +45,8 @@ require (
 	github.com/containerd/errdefs v1.0.0 // indirect
 	github.com/containerd/errdefs/pkg v0.3.0 // indirect
 	github.com/containerd/log v0.1.0 // indirect
-	github.com/containerd/platforms v0.2.1 // indirect
+	github.com/containerd/platforms v1.0.0-rc.2 // indirect
+	github.com/containerd/typeurl/v2 v2.2.3 // indirect
 	github.com/cpuguy83/dockercfg v0.3.2 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/distribution/reference v0.6.0 // indirect
@@ -55,6 +59,7 @@ require (
 	github.com/go-logr/logr v1.4.3 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-ole/go-ole v1.3.0 // indirect
+	github.com/gogo/protobuf v1.3.2 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
@@ -71,6 +76,7 @@ require (
 	github.com/minio/md5-simd v1.1.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect
 	github.com/moby/go-archive v0.2.0 // indirect
+	github.com/moby/locker v1.0.1 // indirect
 	github.com/moby/moby/api v1.55.0 // indirect
 	github.com/moby/moby/client v0.5.0 // indirect
 	github.com/moby/patternmatcher v0.6.1 // indirect
@@ -81,8 +87,6 @@ require (
 	github.com/muesli/ansi v0.0.0-20230316100256-276c6243b2f6 // indirect
 	github.com/muesli/cancelreader v0.2.2 // indirect
 	github.com/muesli/termenv v0.16.0 // indirect
-	github.com/opencontainers/go-digest v1.0.0 // indirect
-	github.com/opencontainers/image-spec v1.1.1 // indirect
 	github.com/philhofer/fwd v1.2.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/power-devops/perfstat v0.0.0-20240221224432-82ca36839d55 // indirect
@@ -108,5 +112,6 @@ require (
 	golang.org/x/oauth2 v0.36.0 // indirect
 	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/text v0.40.0 // indirect
+	google.golang.org/protobuf v1.36.10 // indirect
 	gopkg.in/ini.v1 v1.67.2 // indirect
 )