Add SortedSetCount to IGarnetApi

[badrishc]

We already have SortedSetCount exposed via network as ZCOUNT but there was no way to access it programmatically from the custom stored procedure.

This PR adds the SortedSetCount API to IGarnetApi, and uses it in a sample test custom transaction (stored proc) test/Garnet.test/Extensions/SortedSetCountTxn.cs.

We can also use this PR as a template for how to add API calls to IGarnetApi in future:

• Add method signature to IGarnetApi or IGarnetReadApi interface
• Add implementation of API to StorageSession (e.g. SortedSetOps.cs)
• Add forwarder methods to GarnetApi and GarnetWatchApi (if it is a read operation) struct wrappers

Fixes #979