add checkout middleware (#342)

Implement a connect service middleware which allows an operator to forcefully un-enroll a device. 

Documentation: https://github.com/micromdm/micromdm/wiki/Terminating-a-Management-Relationship-with-a-device-block

platform/remove/internal/removeproto/remove.go

@@ -0,0 +1,3 @@
+package removeproto
+
+//go:generate protoc --go_out=. remove.proto

platform/remove/internal/removeproto/remove.pb.go

@@ -0,0 +1,59 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// source: remove.proto
+
+/*
+Package removeproto is a generated protocol buffer package.
+
+It is generated from these files:
+	remove.proto
+
+It has these top-level messages:
+	Device
+*/
+package removeproto
+
+import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
+import math "math"
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
+type Device struct {
+	Udid string `protobuf:"bytes,1,opt,name=udid" json:"udid,omitempty"`
+}
+
+func (m *Device) Reset()                    { *m = Device{} }
+func (m *Device) String() string            { return proto.CompactTextString(m) }
+func (*Device) ProtoMessage()               {}
+func (*Device) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{0} }
+
+func (m *Device) GetUdid() string {
+	if m != nil {
+		return m.Udid
+	}
+	return ""
+}
+
+func init() {
+	proto.RegisterType((*Device)(nil), "removeproto.Device")
+}
+
+func init() { proto.RegisterFile("remove.proto", fileDescriptor0) }
+
+var fileDescriptor0 = []byte{
+	// 75 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0xe2, 0xe2, 0x29, 0x4a, 0xcd, 0xcd,
+	0x2f, 0x4b, 0xd5, 0x2b, 0x28, 0xca, 0x2f, 0xc9, 0x17, 0xe2, 0x86, 0xf0, 0xc0, 0x1c, 0x25, 0x19,
+	0x2e, 0x36, 0x97, 0xd4, 0xb2, 0xcc, 0xe4, 0x54, 0x21, 0x21, 0x2e, 0x96, 0xd2, 0x94, 0xcc, 0x14,
+	0x09, 0x46, 0x05, 0x46, 0x0d, 0xce, 0x20, 0x30, 0x3b, 0x89, 0x0d, 0xac, 0xc8, 0x18, 0x10, 0x00,
+	0x00, 0xff, 0xff, 0xf6, 0xb9, 0x87, 0xa4, 0x41, 0x00, 0x00, 0x00,
+}

platform/remove/internal/removeproto/remove.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+package removeproto;
+
+message Device {
+    string udid = 1;
+}