docs: Add godocs comment to StateStoreChunkSizeSetter interface

To summarize, we don't really need the `SetStateStoreChunkSize` method, and instead methods like `(*GRPCProvider).ConfigureStateStore` in the `plugin6` package can directly inspect the negotiation process that passes through that code and pull out the chunk size. However that means that that code would also need to implement validation. And that's just `(*GRPCProvider).ConfigureStateStore`; what about all the test mocks that are used in different types of test? They would all need to be implemented similarly to GRPCProvider to be good, useful mocks, and then a lot of things that fulfil the provider.Interface interface are coupled to each other. Instead, it's easier to have validation in the `grpcClient` struct's methods in the `remote` package, as that code is common to all scenarios. That code needs a method to 'reach into'  the provider.Interface value, so we use the `SetStateStoreChunkSize` method.

Author: SarahFrench

internal/providers/provider.go

@@ -151,6 +151,15 @@ type Interface interface {
 	Close() error
 }
 
+// StateStoreChunkSizeSetter interface indicates that a struct wants to record
+// the negotiated chunk size (from the ConfigureStateStore RPC) internally for
+// future use. The future use is likely to be ReadStateBytes/WriteStateBytes RPCs.
+//
+// We let calling code set the chunk size on that struct from outside, to ensure that
+// the value is persisted. The alternative is relying on anything that might fulfil the
+// providers.Interface interface (mock providers used in integration tests, grpcwrap
+// logic used in E2E tests, GRPCProvider logic) to know it needs to implement
+// stateful-ness when processing chunk size negotiation in the ConfigureStateStore RPC.
 type StateStoreChunkSizeSetter interface {
 	SetStateStoreChunkSize(typeName string, size int)
 }