Expose the batch endpoint via the SDK

[jasonjoh]

Is your feature request related to a problem? Please describe the problem.

The core library added support for batching in microsoftgraph/msgraph-sdk-typescript-core#354. However, it is difficult to use as GraphServiceClient exposes no request builders for sending batch requests. As it stands now, you have to create a BatchRequestContent or BatchRequestContentCollection object, passing in a request adapter and error mappings, neither of which are exposed by the client for re-use. This breaks from the approach of other Kiota-based SDKs.

Describe the solution you'd like.

Devs should be able to follow a pattern similar to the .NET SDK, similiar to:

const getMeRequest = graphClient.me.toGetRequestInformation();

const batchRequest = new BatchRequestContent(graphClient);
batchRequest.addBatchRequestStep(getMeRequest);

const response = await graphClient.batch.post(batchRequest);

Additional context?

No response