Update PNR protobuf to support mutable and immutable PNR creation over gRPC

As an PNR gRPC user
I want to be to choose whether immutable or mutable PNR is created in the proto file
So that immutable and mutable PNRs can be created over gRPC 

Given pnr.proto already exists and creates mutable PNRs
When adding immutable PNR support
Then add an additional boolean is_immutable field which defaults to false

Given pnr_handler.rs already exists for handling pnr.proto messages
When adding immutable PNR support over gRPC
Then update create_pnr function to call create_immutable_pnr pnr_service.rs when is_immutable is true
And the existing logic is unchanged when is_immutable is false

Given pnr_service.rs has create_pnr and create_immutable_pnr
When refactoring pnr_service
Then rename create_pnr to create_mutable_pnr for consistency

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