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

setup/config/config_relayapi.go

@@ -0,0 +1,52 @@
+// Copyright 2022 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.
+
+package config
+
+type RelayAPI struct {
+	Matrix *Global `yaml:"-"`
+
+	InternalAPI InternalAPIOptions `yaml:"internal_api,omitempty"`
+	ExternalAPI ExternalAPIOptions `yaml:"external_api,omitempty"`
+
+	// The database stores information used by the relay queue to
+	// forward transactions to remote servers.
+	Database DatabaseOptions `yaml:"database,omitempty"`
+}
+
+func (c *RelayAPI) Defaults(opts DefaultOpts) {
+	if !opts.Monolithic {
+		c.InternalAPI.Listen = "http://localhost:7775"
+		c.InternalAPI.Connect = "http://localhost:7775"
+		c.ExternalAPI.Listen = "http://[::]:8075"
+		c.Database.Defaults(10)
+	}
+	if opts.Generate {
+		if !opts.Monolithic {
+			c.Database.ConnectionString = "file:relayapi.db"
+		}
+	}
+}
+
+func (c *RelayAPI) Verify(configErrs *ConfigErrors, isMonolith bool) {
+	if isMonolith { // polylith required configs below
+		return
+	}
+	if c.Matrix.DatabaseOptions.ConnectionString == "" {
+		checkNotEmpty(configErrs, "relay_api.database.connection_string", string(c.Database.ConnectionString))
+	}
+	checkURL(configErrs, "relay_api.external_api.listen", string(c.ExternalAPI.Listen))
+	checkURL(configErrs, "relay_api.internal_api.listen", string(c.InternalAPI.Listen))
+	checkURL(configErrs, "relay_api.internal_api.connect", string(c.InternalAPI.Connect))
+}