Add proto-generated files to opencensus-proto

[justindsmith]

The opencensus-proto repository holds the canonical reference proto files for types as well as services. We are beginning to have a need to use those protos to communicate with the OCAgent (#124, #128) and need to use those protos to communicate.

There is some work in #128 to move the proto files into the opencensus-node repo (with associated types), but that work should:

1. Live in the opencensus-proto repo to be closer to the source
2. Should be auto-generated instead of manually typed.

Using those generated types where appropriate internally will lead to easier integration with the broader set of OC implementations down the line.

/cc @songy23