Refactor all the types in azure_core to match typespec_client_core

[heaths]

When refactoring most of the general types into typespec and typespec_client_core, I did my best to keep azure_core looking like it did to avoid an even bigger diff with all the changes to imports in other crates. All those other crates also helped double check my progress. Moving those types around became a lower priority.

After beta.1, I'd like to move those types around to match typespec_client_core. I'm pretty happy with the layout - though, open to suggestions otherwise - and I think re-exporting from azure_core that the emitter uses makes sense. Azure service partners will interface entirely with azure_core - which does and will continue to have some Azure-specific types like pagers and pollers - and it gives us flexibility as well.

This will no doubt require changes in the emitter, but my hope is that few to no type names will need to change - just their module paths.

[heaths]

/cc @RickWinter @JeffreyRichter @jhendrixMSFT

[heaths]

Need to also move request and response modules up into http directly. Move Pager to azure_core.