config: sql section for instance config

This section describes the instance SQL configuration.

Part of #8778

NO_DOC=will be added later
NO_CHANGELOG=will be added later

src/box/lua/config/instance_config.lua

@@ -354,6 +354,13 @@ return schema.new('instance_config', schema.record({
             default = false,
         }),
     }),
+    sql = schema.record({
+        cache_size = schema.scalar({
+            type = 'integer',
+            box_cfg = 'sql_cache_size',
+            default = 5 * 1024 * 1024,
+        }),
+    }),
 }, {
     -- Any configuration data should contain a version of the
     -- config schema for which it is written.

test/config-luatest/instance_config_schema_test.lua

@@ -266,3 +266,19 @@ g.test_database = function()
     local res = instance_config:apply_default({}).database
     t.assert_equals(res, exp)
 end
+
+g.test_sql = function()
+    local iconfig = {
+        sql = {
+            cache_size = 1,
+        },
+    }
+    instance_config:validate(iconfig)
+    validate_fields(iconfig.sql, instance_config.schema.fields.sql)
+
+    local exp = {
+        cache_size = 5242880,
+    }
+    local res = instance_config:apply_default({}).sql
+    t.assert_equals(res, exp)
+end