[Service Bus] Remove dead code and add an AMQP constants module

## Summary

The crate carries dead code, and the AMQP operation names, annotation keys, and property keys are written as string literals at their use sites.

## Motivation

Two open items on #2293 record this. Literal operation names are how `com.microsoft:defer-message` entered the crate and stayed there, because nothing collects the names in one place where a reader can compare them against the documented set.

`azure-sdk-for-net` keeps them in `ManagementConstants.cs` and `AmqpMessageConstants.cs`.

## Proposal

- Add a constants module holding the operation names, the annotation keys, and the property keys.
- Replace the string literals at their use sites.
- Remove the dead code the review identified.

Sub-issue of #4934.

Start by comparing the AMQP constants organization in azure-sdk-for-net's ManagementConstants.cs and AmqpMessageConstants.cs, then locate the Rust crate's operation-name, annotation-key, and property-key string literals and the dead code identified in the review. Done means a constants module holds the documented names, all use sites reference it, and the identified dead code is removed.

Written by the indexing model from the issue text.