Refactor publisher proto definition

[devkelley]

Motivation and Context

This change separates out the publisher.proto file into two distinct files. This allows for publishers to implement the callback method without being forced into implementing a 'get_subscription_info' call. That call will be dependent on each publisher and may vary slightly so it has been removed from the required proto.

The 'get_subscription_info' proto definition was moved into the sample code to demonstrate how a publisher could surface the dynamically created topics to a subscriber.

Description

This PR makes the following changes:

• Separates publisher.proto into two distinct services, 'PublisherCallback' (required to implement for a publisher) and 'SamplePublisher' (an example of how a publisher could surface dynamic topics.
• Removes from the constants file a publisher reference and places it in the samples_settings file.
• Adds 'samples_proto' to the samples folder, and places the 'sample_publisher.proto' file in there.
• Samples now utilize the 'samples_proto' crate over the proto crate used by the pub sub service.
• Samples show how to initialize a publisher with two grpc services (PublisherCallback and SamplePublisher) while sharing the topic store information.

NOTE: The 'publisher_impl.rs' files in chariott-publisher and simple-publisher are the exact same.

Additionally:

• Modifies 'topic_store' in samples to be easier to interact with, allowing for the base 'Publisher' class to be cloned.

[ladatz]

Please consider my comments, just a few suggestions

[ashbeitz]

Just one comment.

[ashbeitz]

Approved.