config: add isolated option on the instance level

This commit just adds the option into the cluster configuration schema.
Next commits will add the relevant logic.

Part of tarantool#10796

NO_DOC=tarantool/doc#4632

Author: Totktonada

changelogs/unreleased/config-isolated-instance.md

@@ -0,0 +1,3 @@
+## feature/config
+
+* Add `isolated` option for instances (gh-10796).

src/box/lua/config/instance_config.lua

@@ -2475,6 +2475,12 @@ return schema.new('instance_config', schema.record({
             type = 'string',
         }),
     }),
+    isolated = schema.scalar({
+        type = 'boolean',
+        default = false,
+        -- TODO: verify that the option is set in instance scope
+        -- or using an environment variable.
+    }),
 }, {
     -- This kind of validation cannot be implemented as the
     -- 'validate' annotation of a particular schema node. There

test/config-luatest/cluster_config_schema_test.lua

@@ -92,6 +92,7 @@ local instance_config_fields = {
     'failover',
     'compat',
     'labels',
+    'isolated',
 }
 
 -- Verify that the fields of the given schema correspond to the
@@ -427,6 +428,7 @@ g.test_defaults = function()
             console_session_scope_vars = 'old',
             wal_cleanup_delay_deprecation = 'old',
         },
+        isolated = false,
     }
 
     -- Global defaults.

test/config-luatest/instance_config_schema_test.lua

@@ -1764,3 +1764,8 @@ g.test_labels = function()
 
     t.assert_equals(instance_config:apply_default({}).labels, nil)
 end
+
+g.test_isolated = function()
+    instance_config:validate({isolated = true})
+    t.assert_equals(instance_config:apply_default({}).isolated, false)
+end