Break up PolykeyAgent and PolykeyClient classes based on domain models

[robert-cronin]

Right now there are 2 large files; one for the agent and one for the client. This is not very cohesive and needs to be split into separate files based on some good domain modelling.

[robert-cronin]

PolykeyClient is now superseded by a codegen client. The agent file remains one big file however

[robert-cronin]

This is probably less ideal to break up into different sub files since the class needs to conform to the interface generated by gRPC.
I tried doing it initially, but it just didn't really seem to make it better.