Expose the batch endpoint via the SDK

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

The core library added support for batching in https://github.com/microsoftgraph/msgraph-sdk-typescript-core/pull/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

Start with GraphServiceClient and the core library's BatchRequestContent and BatchRequestContentCollection APIs, then compare the analogous .NET SDK pattern described in the issue. Done means callers can construct batch content with graphClient, add request steps, and submit it through graphClient.batch.post without separately providing the request adapter or error mappings.