The gRPC protobuf will be shared by EOS, CTA and CERNBox, so it needs to
go in its own submodule.

- CASTOR migration tools stay under CTA/migration
- upgrade tools are now under CTA/upgrade_db
- adds FindGRPC.cmake (required for migration tools)
- updates submodule to v0.50 (required for EOS gRPC API)

Default now set to 10s. Note that XRD_TIMEOUTRESOLUTION must be set
<= 10s, otherwise timeout defaults to 15s.

Now the submodule does not provide a CMakeLists.txt file, this is now
provided by the main project (eos_cta/CMakeLists.txt)