As a PNR gRPC API consumer
I want to be able to easily add or update individual PNR Records within a PNR Zone
So that I don't have to provide a full PNR Record with all the PNR zones every time I make a modification

Given pnr_handler.rs provides PNR operations
When adding or updating a PNR Record
Then add an UpdatePnrRecord RPC to add or update PNR Records (with key {record}) on an existing PNR Zone (with key {name})
And update pnr.proto in a similar way to the example below:

```
rpc UpdatePnrRecord(UpdatePnrRecordRequest) returns (PnrResponse);

message UpdatePnrRecordRequest {
  string name = 1;
  string record = 2;
  PnrRecord pnr_record = 3;
  optional string store_type = 4;
}
```

Given pnr_service.rs performs changes to PNR zones and records
When adding an PNR record to an existing PNR zone
Then use update_pnr_record function in pnr_service.rs to add/update the PNR record

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)
- Update unit tests to validate the changes
- Update lib.rs with the new PNR function if needed
