[azservicebus] Recreate the ATOM client using the autorest generator

[richardpark-msft]

The ATOM client in the azservicebus package was generated from an older client, and is not standardized like the rest of our autogenerated libraries (for instance, it has it's own "custom" retry policy, etc..).

We need to regenerate this, based on the swagger for the API.

My first pass at this has shown that it does generate some parts of the client correctly. This has been done in Python and Java, so this might be a case of some limitation of our current autorest generator.

Swagger link: sb-swagger

If the generation is correct, it should fix:

• [azservicebus] Allow for recreating pagers in the ATOM API #15972 (make pagers recreatable),
• [azservicebus] Add in support in the AdminClient for MaxMessageSizeInKilobytes #15917 (add support for MaxMessageSizeInKilobytes)
• [servicebus sdk] missing name on entities datamodel returned from admin client #16692 (missing fields in data models)
• [azservicebus] AdminClient - Rule management functions #16045 (manage subscription rules in the ATOM client)

[richardpark-msft]

At this point we're far past GA. Regenerating might have some utility now but it's far less needed than it would have been before. Closing for now.