Add get PNR support to gRPC APIs
As an gRPC API consumer
I want to be able to get PNRs
So that I can view the current state of the PNR zones

Given pnr_handler.rs handles gRPC request for PNRs
When adding GET PNR support
Then update pnr_handler.rs to include a get_pnr function
And use pointer_handler.rs as a reference

Given pnr_service.rs implements get_pnr
When adding GET PNR support
Then call get_pnr in a similar way to pnr_controller.rs

Given gRPC requires protobuffer definitions
When adding get PNR support
Then update gRPC service definition for PNR request/response messages in /proto/pnr.proto

Given gRPC needs to be integrated with Tonic server
When adding get PNR gRPC endpoint
Then update build.rs to include pnr.proto changes

Provide unit tests where applicable for new code.

Implementation Notes

- Place any unit tests at the bottom of the same file as the associated production code
- Increment patch version of anttp package in Cargo.toml
- Create a copy of this issue description and save to /spec using the name of this issue as the filename (with lower underscore case, starting with the 5 char padded issue number, e.g. 00001_issue_title.txt)