Publish Protobuf definitions to Buf Schema Registry

[FZambia]

Proposed changes

Relates #851

With this change Centrifugo will publish its Protobuf definitions to Buf Schema Registry (BSR) on every release. Publishing:

• apiproto - Protobuf to communicate with server GRPC API
• unistream - Protobuf to consume unidirectional GRPC
• proxyproto - Protobuf of Centrifugo GRPC event proxy, in this case Centrifugo itself plays a role of a client.

It's important to mention that this all helps only with GRPC communication. Centrifugo APIs are not compatible with Buf connect's HTTP – we have our custom HTTP implementaion which re-uses Protobuf definitions for requests but still differs from Buf's connect framework. The differences may be eliminated probably, though requires a separate research – the hardest part is that for HTTP we use custom Raw type which is like a json.RawMessage under the hood and allows passing JSON objects without any wrapping.

Example for Go

package main

import (
	"context"
	"fmt"
	"log"

	apigrpc "buf.build/gen/go/centrifugo/apiproto/grpc/go/_gogrpc"
	apiproto "buf.build/gen/go/centrifugo/apiproto/protocolbuffers/go"
	"google.golang.org/grpc"
)

func main() {
	conn, err := grpc.NewClient("localhost:10000")
	if err != nil {
		log.Fatalln(err)
	}
	defer func() { _ = conn.Close() }()

	client := apigrpc.NewCentrifugoApiClient(conn)
	res, err := client.Publish(
		context.Background(),
		&apiproto.PublishRequest{
}