Add initial implementation of Agate snapshot library

Introduces core functionality for the Agate library, including snapshot creation, restoration, listing, and deletion. Adds examples for basic usage, gRPC proto definitions, and build/configuration files such as `go.mod` and `Makefile`. The implementation establishes the framework for store integration and placeholder server functionality.

Makefile

@@ -0,0 +1,16 @@
+download-third-party:
+	rm -rf ./grpc/third_party
+	mkdir -p ./grpc/third_party
+	cd ./grpc/third_party && git clone https://github.com/googleapis/googleapis.git
+	mv ./grpc/third_party/googleapis/google ./grpc
+	mv ./grpc/third_party/googleapis/grafeas ./grpc
+	rm -rf ./grpc/third_party
+
+gen-proto-geolocation:
+	mkdir -p ./grpc
+
+	@protoc -I ./grpc \
+			--go_out=grpc --go_opt paths=source_relative \
+			--go-grpc_out=grpc --go-grpc_opt paths=source_relative \
+			--grpc-gateway_out=grpc --grpc-gateway_opt paths=source_relative \
+			./grpc/snapshot.proto