[MM-53968] Includes mattermost-plugin-api into the mono repo (#24235)

Include https://github.com/mattermost/mattermost-plugin-api into the mono repo

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Michael Kochell <mjkochell@gmail.com>
Co-authored-by: Alejandro García Montoro <alejandro.garciamontoro@gmail.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Alex Dovenmuehle <alex.dovenmuehle@mattermost.com>
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Co-authored-by: Shota Gvinepadze <wineson@gmail.com>
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Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>
Co-authored-by: Daniel Espino García <larkox@gmail.com>
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server/public/model/manifest.go

@@ -191,7 +191,8 @@ type Manifest struct {
 
 	// RequiredConfig defines any required server configuration fields for the plugin to function properly.
 	//
-	// Use the pluginapi.Configuration.CheckRequiredServerConfiguration method to enforce this.
+	// Deprecated: The required server configuration fields should be checked using custom code.
+	// This field will get removed in the next major release.
 	RequiredConfig *Config `json:"required_configuration,omitempty" yaml:"required_configuration,omitempty"`
 }