Refactor(keyvault): migrate to unified folder structure

[v-xuto]

This PR migrates the keyvault specification to the new unified folder structure according to Azure folder structure guidelines.

Change:

• Consolidated TypeSpec and Swagger files under

├── data-plane                   
│   ├── Administration            
│   │   ├── BackupRestore
│   │   ├── RBAC
│   │   └── Settings   
│   ├── Certificates             
│   ├── Keys                     
│   ├── Secrets                   
│   ├── SecretsStorage            
│   ├── SecurityDomain            
│   └── Legacy       # Old api versions
│
└── resource-manager/Microsoft.KeyVault/KeyVault          

• Updated tspconfig.yaml with correct paths and ARM types directory
• Fixed all common-types references and input-file paths in readme.md
• Added suppressions.yaml for legacy API versions
• Verified TypeSpec compilation works correctly
• Updated the relevant paths in specification/suppressions.yaml file
• Updated the relevant paths in cspell.yaml file

Benefits:

• Unified structure for better maintainability
• Aligns with Azure folder structure standards
• Maintains backward compatibility
• Supports future TypeSpec migration efforts

[github-actions]

Next Steps to Merge

Next steps that must be taken to merge this PR:

• ❌ This PR targets either the main branch of the public specs repo or the RPSaaSMaster branch of the private specs repo. These branches are not intended for iterative development. Therefore, you must acknowledge you understand that after this PR is merged, the APIs are considered shipped to Azure customers. Any further attempts at in-place modifications to the APIs will be subject to Azure's versioning and breaking change policies. Additionally, for control plane APIs, you must acknowledge that you are following all the best practices documented by ARM at aka.ms/armapibestpractices. If you do intend to release the APIs to your customers by merging this PR, add the PublishToCustomers label to your PR in acknowledgement of the above. Otherwise, retarget this PR onto a feature branch, i.e. with prefix release- (see aka.ms/azsdk/api-versions#release--branches).
• ❌ Your PR requires an API stewardship board review as it introduces a new API version (label: new-api-version). Schedule the review by following aka.ms/azsdk/onboarding/restapischedule.
• ❌ This PR is NotReadyForARMReview because it has the CI-NewRPNamespaceWithoutRPaaS label.
  
• ❌ This PR has CI-NewRPNamespaceWithoutRPaaS label. This means it is introducing a new RP (Resource Provider) namespace that has not been onboarded with RPaaS. Merging this PR to the main branch is blocked as RPaaS is required for new RPs.
  To resolve, use RPaaS to onboard the new RP. To apply for exception, see aka.ms/RPaaSException.
• ❌ The required check named TypeSpec Validation has failed. Refer to the check in the PR's 'Checks' tab for details on how to fix it and consult the aka.ms/ci-fix guide

Comment generated by summarize-checks workflow run.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
TypeSpec	KeyVault
TypeSpec	Microsoft.KeyVault
Swagger	Administration-Certificates
Swagger	Administration-Keys
Swagger	Administration-Legacy
Swagger	Administration-Secrets
Swagger	Administration-SecretsStorage
Swagger	Administration-SecurityDomain
Swagger	Microsoft.KeyVault-KeyVault
Swagger	Administration-Administration
JavaScript	@azure/keyvault-admin
JavaScript	@azure/keyvault-certificates
JavaScript	@azure/keyvault-keys
JavaScript	@azure/keyvault-secrets
JavaScript	@azure/arm-keyvault
Python	azure-keyvault-administration
Python	azure-keyvault-certificates
Python	azure-keyvault-keys
Python	azure-keyvault-secrets
Python	azure-mgmt-keyvault

[chlowell]

The SDK team is still discussing whether and how to refactor these projects