Make gRCP max message size configurable

Currently, gRPC maximum message size is hard set to 24 MiB at

spark-dgraph-connector/src/main/scala/uk/co/gresearch/spark/dgraph/connector/package.scala

Line 196 in 78cb221

    
           def toChannel(target: Target): ManagedChannel = NettyChannelBuilder.forTarget(target.toString).usePlaintext().maxInboundMessageSize(24 * 1024 * 1024).build()

Make this number configurable via spark.read.option.

Further, distinguish between the channel used in SchemaProvider and DgraphExecutor, and provide individual settings (a general max size and two specific sizes).