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/model/plugin_valid.go

@@ -0,0 +1,40 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"regexp"
+	"unicode/utf8"
+)
+
+const (
+	MinIdLength  = 3
+	MaxIdLength  = 190
+	ValidIdRegex = `^[a-zA-Z0-9-_\.]+$`
+)
+
+// ValidId constrains the set of valid plugin identifiers:
+//
+//	^[a-zA-Z0-9-_\.]+
+var validId *regexp.Regexp
+
+func init() {
+	validId = regexp.MustCompile(ValidIdRegex)
+}
+
+// IsValidPluginId verifies that the plugin id has a minimum length of 3, maximum length of 190, and
+// contains only alphanumeric characters, dashes, underscores and periods.
+//
+// These constraints are necessary since the plugin id is used as part of a filesystem path.
+func IsValidPluginId(id string) bool {
+	if utf8.RuneCountInString(id) < MinIdLength {
+		return false
+	}
+
+	if utf8.RuneCountInString(id) > MaxIdLength {
+		return false
+	}
+
+	return validId.MatchString(id)
+}