feat(neptune): high level constructs for db clusters and instances

- This change adds higher level constructs for Neptune clusters
- Adds higher-level constructs for
  - AWS::Neptune::DBCluster
  - AWS::Neptune::DBInstance
  - AWS::Neptune::DBClusterParameterGroup
  - AWS::Neptune::DBParameterGroup
  - AWS::Neptune::DBSubnetGroup

fixes aws#12762

packages/@aws-cdk/aws-neptune/README.md

@@ -9,10 +9,45 @@
 >
 > [CFN Resources]: https://docs.aws.amazon.com/cdk/latest/guide/constructs.html#constructs_lib
 
+![cdk-constructs: Experimental](https://img.shields.io/badge/cdk--constructs-experimental-important.svg?style=for-the-badge)
+
+> The APIs of higher level constructs in this module are experimental and under active development.
+> They are subject to non-backward compatible changes or removal in any future version. These are
+> not subject to the [Semantic Versioning](https://semver.org/) model and breaking changes will be
+> announced in the release notes. This means that while you may use them, you may need to update
+> your source code when upgrading to a newer version of this package.
+
 ---
 
 <!--END STABILITY BANNER-->
 
+## Starting a Neptune Database
+
+To set up a Neptune database, define a `DatabaseCluster`. You must always launch a database in a VPC. 
+
+```ts
+const cluster = new DatabaseCluster(this, 'Database', {
+    vpc,
+    instanceProps: {
+        instanceType: InstanceType.R5_LARGE,
+    }
+});
+```
+
+Your cluster will be empty by default.
+
+## Connecting
+
+To control who can access the cluster, use the `.connections` attribute. Neptune databases have a default port, so
+you don't need to specify the port:
+
+```ts
+cluster.connections.allowDefaultPortFromAnyIpv4('Open to the world');
+```
+
+The endpoints to access your database cluster will be available as the `.clusterEndpoint` and `.clusterReadEndpoint`
+attributes:
+
 ```ts
-import * as neptune from '@aws-cdk/aws-neptune';
+const writeAddress = cluster.clusterEndpoint.socketAddress;   // "HOSTNAME:PORT"
 ```

packages/@aws-cdk/aws-neptune/lib/cluster-ref.ts

@@ -0,0 +1,75 @@
+import { IConnectable, ISecurityGroup } from '@aws-cdk/aws-ec2';
+import { IResource } from '@aws-cdk/core';
+import { Endpoint } from './endpoint';
+
+/**
+ * Create a clustered database with a given number of instances.
+ */
+export interface IDatabaseCluster extends IResource, IConnectable {
+  /**
+   * Identifier of the cluster
+   */
+  readonly clusterIdentifier: string;
+
+  /**
+   * Identifiers of the replicas
+   */
+  readonly instanceIdentifiers: string[];
+
+  /**
+   * The endpoint to use for read/write operations
+   * @attribute Endpoint,Port
+   */
+  readonly clusterEndpoint: Endpoint;
+
+  /**
+   * Endpoint to use for load-balanced read-only operations.
+   * @attribute ReadEndpoint
+   */
+  readonly clusterReadEndpoint: Endpoint;
+
+  /**
+   * Endpoints which address each individual replica.
+   */
+  readonly instanceEndpoints: Endpoint[];
+}
+
+/**
+ * Properties that describe an existing cluster instance
+ */
+export interface DatabaseClusterAttributes {
+  /**
+   * The database port
+   */
+  readonly port: number;
+
+  /**
+   * The security group of the database cluster
+   */
+  readonly securityGroup: ISecurityGroup;
+
+  /**
+   * Identifier for the cluster
+   */
+  readonly clusterIdentifier: string;
+
+  /**
+   * Identifier for the instances
+   */
+  readonly instanceIdentifiers: string[];
+
+  /**
+   * Cluster endpoint address
+   */
+  readonly clusterEndpointAddress: string;
+
+  /**
+   * Reader endpoint address
+   */
+  readonly readerEndpointAddress: string;
+
+  /**
+   * Endpoint addresses of individual instances
+   */
+  readonly instanceEndpointAddresses: string[];
+}