[Session] Adding the Session type (#54)

## Summary

Scaffolding & adding the `Session` type

proto/pocket/session/session.proto

@@ -0,0 +1,36 @@
+syntax = "proto3";
+package pocket.session;
+
+option go_package = "pocket/x/session/types";
+
+import "cosmos_proto/cosmos.proto";
+// TODO(@Olshansk): Uncomment the line below once the service.proto file is added
+// import "pocket/service/service.proto";
+import "pocket/application/application.proto";
+import "pocket/supplier/supplier.proto";
+
+// NOTE: Using signed integers for consistency with the cosmos SDK
+
+// SessionHeader is a lightweight header for a session that can be passed around.
+// It is the minimal amount of data required to hydrate & retrieve all data relevant to the session.
+message SessionHeader {
+    string application_address = 1 [(cosmos_proto.scalar) = "cosmos.AddressString"]; // The address of the application this session is for
+    // TODO(@Olshansk): Uncomment the line below once the `ServiceId` proto is defined
+    // service.ServiceId service_id = 2; // The ID of the service this session is servicing
+    int64 session_start_block_height = 3; // The height at which this session started
+    // NOTE: session_id can be derived from the above values using on-chain but is included in the header for convenience
+    string session_id = 4; // A unique pseudoranom ID for this session
+}
+
+// Session is a fully hydrated session object that contains all the information for the Session
+// and its parcipants.
+message Session {
+    SessionHeader header = 1; // The header of the session containing lightweight data
+    string session_id = 2; // A unique pseudoranom ID for this session
+    int64 session_number = 3; // The session number since genesis
+    int64 num_blocks_per_session = 4;  // The number of blocks per session when this session started
+    // TODO(@Olshansk): Uncomment the line below once the `Service` proto is defined
+    // service.Service service = 5; // A fully hydrated service object this session is for
+    application.Application application = 6; // A fully hydrated application object this session is for
+    repeated supplier.Supplier suppliers = 7; // A fully hydrated set of servicers that are serving the application
+}