This is an initial partial description of the plugin protocol focused
mainly on explaining the purpose of the .proto files. In subsequent
updates we will also document the negotiation protocol, etc.
For this first pass the goal was just to publish some information that was
already available in an internal design document so that it's visible to
SDK implementers. It is focused on the .proto versioning strategy because
that was the main topic of the internal design documentation this was
based on.
Add versioned tfplugin proto files to the docs directory, for easier
reference. The latest version starts as a symlink to the current
file used for generated the tfplugin package in ./internal/tfplugin5.
When changing the protocol version, the old file must be copied to
./docs/plugin-protocol/, and a new symlink created for the latest
version.
