Make api a Go sub-module

Allow the api to stay at the same v1 go package name and keep using a
1.x version number. This indicates the API is still at 1.x and allows
sharing proto types with containerd 1.6 and 1.7 releases.

Signed-off-by: Derek McGowan <derek@mcg.dev>

api/go.mod

@@ -0,0 +1,22 @@
+module github.com/containerd/containerd/api
+
+go 1.22.0
+
+require (
+	github.com/containerd/ttrpc v1.2.3
+	github.com/containerd/typeurl/v2 v2.1.1
+	github.com/opencontainers/image-spec v1.1.0
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20240401170217-c3f982113cda
+	google.golang.org/grpc v1.59.0
+	google.golang.org/protobuf v1.33.0
+)
+
+require (
+	github.com/gogo/protobuf v1.3.2 // indirect
+	github.com/golang/protobuf v1.5.3 // indirect
+	github.com/opencontainers/go-digest v1.0.0 // indirect
+	github.com/sirupsen/logrus v1.8.1 // indirect
+	golang.org/x/net v0.21.0 // indirect
+	golang.org/x/sys v0.17.0 // indirect
+	golang.org/x/text v0.14.0 // indirect
+)