Initial Store & Forward Implementation (#2917)

This adds store & forward relays into dendrite for p2p.
A few things have changed:
- new relay api serves new http endpoints for s&f federation
- updated outbound federation queueing which will attempt to forward
using s&f if appropriate
- database entries to track s&f relays for other nodes

federationapi/storage/shared/receipt/receipt.go

@@ -0,0 +1,42 @@
+// Copyright 2023 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// A Receipt contains the NIDs of a call to GetNextTransactionPDUs/EDUs.
+// We don't actually export the NIDs but we need the caller to be able
+// to pass them back so that we can clean up if the transaction sends
+// successfully.
+
+package receipt
+
+import "fmt"
+
+// Receipt is a wrapper type used to represent a nid that corresponds to a unique row entry
+// in some database table.
+// The internal nid value cannot be modified after a Receipt has been created.
+// This guarantees a receipt will always refer to the same table entry that it was created
+// to represent.
+type Receipt struct {
+	nid int64
+}
+
+func NewReceipt(nid int64) Receipt {
+	return Receipt{nid: nid}
+}
+
+func (r *Receipt) GetNID() int64 {
+	return r.nid
+}
+
+func (r *Receipt) String() string {
+	return fmt.Sprintf("%d", r.nid)
+}