feat: add client constructor with executor (DGRAPH-2746)

[abhimanyusinghgaur]

Fixes DGRAPH-2746
Fixes Discuss Issue

This PR adds a constructor to DgraphAsyncClient which accepts a java.util.concurrent.Executor argument to be used to process various asynchronous tasks executed by the client. This is useful in scenarios where one wants to do resource management manually, and not use the default pool provided by java.util.concurrent.ForkJoinPool.commonPool().

It also adds a similar constructor to the synchronous version of the client: DgraphClient.

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This change is 

[lhr0909]

Overall LGTM. I am just curious if we need to put the executor under the thenAccept usages around the login methods.

[abhimanyusinghgaur]

Currently, those are synchronous computations that will be executed either by gRPC's executor (which can be configured while constructing a ManagedChannel for building DgraphStub) or by the thread which calls get() on the CompletableFuture.

Those computations being very small, there doesn't seem to be a need to make them async.