Provide batch position and size information inside entity operations

[sebastianburckhardt]

Adds two properties to IDurableEntityContext:

        /// <summary>
        /// The size of the current batch of operations.
        /// </summary>
        int BatchSize { get; }

        /// <summary>
        /// The position of the currently executing operation within the current batch of operations.
        /// </summary>
        int BatchPosition { get; }

as discussed in #1456.

[ConnorMcMahon]

LGTM!

@davidmrdavid, let's file issues against the Durable JS and Durable Python repos to add this, as it should be very straightforward.

[davidmrdavid]

This is pretty cool! I'll file the issues @ConnorMcMahon and link them here

[davidmrdavid]

JS: Azure/azure-functions-durable-js#226
Python: Azure/azure-functions-durable-python#227

Also, funny how the issue numbers between the two repos are almost in sync haha. 227 and 226, interesting....