Expose public/ API as submodule (#23345)

* model -> public/model

* plugin -> public/plugin

* public/model/utils -> public/utils

* platform/shared/mlog -> public/shared/mlog

* platform/shared/i18n -> public/shared/i18n

* platform/shared/markdown -> public/shared/markdown

* platform/services/timezones -> public/shared/timezones

* channels/einterfaces -> einterfaces

* expose public/ submodule

* go mod tidy

* .github: cache-dependency-path, setup-go-work

* modules-tidy for public/ too

* remove old gomodtidy

server/public/plugin/io_rpc.go

@@ -0,0 +1,46 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin
+
+import (
+	"bufio"
+	"encoding/binary"
+	"io"
+)
+
+type remoteIOReader struct {
+	conn io.ReadWriteCloser
+}
+
+func (r *remoteIOReader) Read(b []byte) (int, error) {
+	var buf [10]byte
+	n := binary.PutVarint(buf[:], int64(len(b)))
+	if _, err := r.conn.Write(buf[:n]); err != nil {
+		return 0, err
+	}
+	return r.conn.Read(b)
+}
+
+func (r *remoteIOReader) Close() error {
+	return r.conn.Close()
+}
+
+func connectIOReader(conn io.ReadWriteCloser) io.ReadCloser {
+	return &remoteIOReader{conn}
+}
+
+func serveIOReader(r io.Reader, conn io.ReadWriteCloser) {
+	cr := bufio.NewReader(conn)
+	defer conn.Close()
+	buf := make([]byte, 32*1024)
+	for {
+		n, err := binary.ReadVarint(cr)
+		if err != nil {
+			break
+		}
+		if written, err := io.CopyBuffer(conn, io.LimitReader(r, n), buf); err != nil || written < n {
+			break
+		}
+	}
+}