feat(sql): update the API

#### sql:v1beta4
The following keys were added:
- schemas.ExportContext.properties.offload.description
- schemas.ExportContext.properties.offload.type

The following keys were changed:
- schemas.DatabaseInstance.properties.databaseVersion.enum
- schemas.DatabaseInstance.properties.databaseVersion.enumDescriptions
- schemas.Flag.properties.appliesTo.enumDescriptions
- schemas.Flag.properties.appliesTo.items.enum

discovery/sql-v1beta4.json

@@ -1818,7 +1818,7 @@
       }
     }
   },
-  "revision": "20200331",
+  "revision": "20200518",
   "rootUrl": "https://sqladmin.googleapis.com/",
   "schemas": {
     "AclEntry": {
@@ -2181,7 +2181,8 @@
             "SQLSERVER_2017_EXPRESS",
             "SQLSERVER_2017_WEB",
             "POSTGRES_10",
-            "POSTGRES_12"
+            "POSTGRES_12",
+            "MYSQL_8_0"
           ],
           "enumDescriptions": [
             "This is an unknown database version.",
@@ -2196,7 +2197,8 @@
             "The database version is SQL Server 2017 Express.",
             "The database version is SQL Server 2017 Web.",
             "The database version is PostgreSQL 10.",
-            "The database version is PostgreSQL 12."
+            "The database version is PostgreSQL 12.",
+            "The database version is MySQL 8."
           ],
           "type": "string"
         },
@@ -2525,6 +2527,10 @@
           "description": "This is always <code>sql#exportContext</code>.",
           "type": "string"
         },
+        "offload": {
+          "description": "Option for export offload.",
+          "type": "boolean"
+        },
         "sqlExportOptions": {
           "description": "Options for exporting data as SQL statements.",
           "properties": {
@@ -2610,7 +2616,8 @@
             "The database version is SQL Server 2017 Express.",
             "The database version is SQL Server 2017 Web.",
             "The database version is PostgreSQL 10.",
-            "The database version is PostgreSQL 12."
+            "The database version is PostgreSQL 12.",
+            "The database version is MySQL 8."
           ],
           "items": {
             "enum": [
@@ -2626,7 +2633,8 @@
               "SQLSERVER_2017_EXPRESS",
               "SQLSERVER_2017_WEB",
               "POSTGRES_10",
-              "POSTGRES_12"
+              "POSTGRES_12",
+              "MYSQL_8_0"
             ],
             "type": "string"
           },

src/apis/sql/v1beta4.ts

@@ -628,6 +628,10 @@ export namespace sql_v1beta4 {
      * This is always <code>sql#exportContext</code>.
      */
     kind?: string | null;
+    /**
+     * Option for export offload.
+     */
+    offload?: boolean | null;
     /**
      * Options for exporting data as SQL statements.
      */