Fix linter errors and add linting to CI

[Unique-Divine]

Current linter errors when running

golangci-lint run --allow-parallel-runners --fix

feeder/eventstream/stream.go:27:22: SA1019: grpc.WithInsecure is deprecated: use WithTransportCredentials and insecure.NewCredentials() instead. Will be supported throughout 1.x. (staticcheck)
	transportDialOpt := grpc.WithInsecure()
	                    ^
feeder/eventstream/stream_test.go:60:39: SA1019: grpc.WithInsecure is deprecated: use WithTransportCredentials and insecure.NewCredentials() instead. Will be supported throughout 1.x. (staticcheck)
	conn, err := grpc.Dial(grpcEndpoint, grpc.WithInsecure())
	                                     ^
feeder/priceposter/client.go:58:22: SA1019: grpc.WithInsecure is deprecated: use WithTransportCredentials and insecure.NewCredentials() instead. Will be supported throughout 1.x. (staticcheck)
	transportDialOpt := grpc.WithInsecure()
	                    ^
cmd/root.go:72:22: Error return value of `http.ListenAndServe` is not checked (errcheck)
		http.ListenAndServe(":8080", nil)
		                   ^
exit status 1

Commit

6f1804f