Remove dependency on OpenAPI for Arm Specs

[iscai-msft]

Clear and concise description of the problem

Right now, the ARM library depends on typespec-autorest, I believe only for their @extension headers. This creates issues becausetcgc can not test ARM code because typespec-autorest depends on tcgc. Additionally, we should also be moving away from the @extension decorator.

I've taken the liberty to compile a list of the extensions I see in use in the main branch of azure-rest-api-specs

• x-ms-azure-resource
• x-ms-identifiers
• x-ms-client-flatten use @azure-tools/typespec-client-generator-core's @flattenProperty instead
• x-ms-long-running-operation use LRO framework instead in azure-core
• x-ms-long-running-operation-options use LRO framework instead in azure-core

Tasks

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1. Removing dependency on typespec-autorest from arm package #697
   needs-area +1
   
2. x-ms-azure-resource
3. x-ms-identifiers alternative #1473
   design:accepted lib:azure-resource-manager +2
   
4. x-ms-client-flatten use @azure-tools/typespec-client-generator-core's @flattenProperty instead
5. x-ms-long-running-operation use LRO framework instead in azure-core
6. x-ms-long-running-operation-options use LRO framework instead in azure-core