Split codegen_operations into multiple files

[krishanjmistry]

Personal opinion: it's quite difficult to navigate through the codegen_[operations/models] files.
The changes made here split codegen_operations.rs into several files.

There was also some duplications DocCommentCode that I noticed - now operations and models use the same

Tested by running azure-autorust. No changes found to any of the autogenerated crates.

Things noticed:

• there's some unused code - I've added #[allow(dead_code)] where necessary
• AuthCode is only used in one spot - but we're adding the authorization header in 2 other places within request_builder_into_future.rs. Worth noting in other SDK's there's an AuthenticationPolicy:
  • https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/core/corehttp/corehttp/runtime/policies/_authentication.py
  • https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/core/Azure.Core/src/Pipeline/BearerTokenAuthenticationPolicy.cs

[cataggar]

Sure, I'm okay with splitting it up.