Add/update scripts for vendoring dependencies (#396)

Updating vendored dependencies is a lengthy process at the moment. This PR:
- Adds a new `vendor-all.sh` script that allows one to upgrade all vendored dependencies (gRPC Core, nanopb, BoringSSL) by simply calling:

```
cd scripts
./vendor-all.sh v1.14.0
```

- Moves other vendoring scripts into `scripts/`
- Updates temporary paths used for cloning `grpc` to use `scripts/tmp` instead of `third_party`

.gitignore

@@ -12,6 +12,7 @@ third_party/**
 /test.out
 /echo.pid
 /SwiftGRPC.xcodeproj
+/scripts/tmp/
 Examples/EchoWeb/dist
 Examples/EchoWeb/node_modules
 Examples/EchoWeb/package-lock.json