feat(config): add acpPluginInstallDisabled option (#13189)

public/language/en-GB/error.json

@@ -268,6 +268,7 @@
 
 	"invalid-plugin-id": "Invalid plugin ID",
 	"plugin-not-whitelisted": "Unable to install plugin – only plugins whitelisted by the NodeBB Package Manager can be installed via the ACP",
+	"plugin-installation-via-acp-disabled": "Plugin installation via ACP is disabled",
 	"plugins-set-in-configuration": "You are not allowed to change plugin state as they are defined at runtime (config.json, environmental variables or terminal arguments), please modify the configuration instead.",
 	"theme-not-set-in-configuration": "When defining active plugins in configuration, changing themes requires adding the new theme to the list of active plugins before updating it in the ACP",
 

public/openapi/read/admin/development/info.yaml

@@ -98,6 +98,8 @@ get:
                               type: boolean
                             jobsDisabled:
                               type: boolean
+                            acpPluginInstallDisabled:
+                              type: boolean
                         git:
                           type: object
                           properties: