Use enumeration for LibraryLocation and LibraryLayout fields in grpc API

[cmaglie]

The enumeration is also reused inside the arduino package to reduce code duplication.

This kind of change IMHO improves:

• code duplication, because the enumeration is defined in a single place
• the old string field in grpc definition, because now is an enumeraion and not a free string: t's more clear which value it can assume
• language bindings. In particular in the Java bindings the generated API is better, for example see arduino/Arduino@9f58bae

As a side effect, after this change, the rpc definitions starts to spill inside the arduino package.
This means that the rpc definitions inside the .proto files became a sort of tables-of-truth, at least for this part of the API.

[cmaglie]

Superseded by #613