Add Azure Key Vault environment repository

[yashdotdev13]

Description

Adds Azure Key Vault support as an environment repository for Spring Cloud Config Server.

Changes

• Added AzureKeyVaultEnvironmentRepository
• Added Azure Key Vault configuration properties
• Added repository factory and auto-configuration support
• Added Azure SDK dependencies and BOM management
• Added support for application and profile-based secret resolution
• Added property name conversion and configurable separators
• Added repository ordering support
• Added Azure Key Vault backend documentation and navigation entry
• Added unit tests for the Azure Key Vault environment repository

Testing

• .\mvnw.cmd -pl spring-cloud-config-server -Dtest=AzureKeyVaultEnvironmentRepositoryTests test
• .\mvnw.cmd -pl spring-cloud-config-server -DskipTests compile
• git diff --check

All targeted Azure Key Vault tests pass successfully.

Full Test Suite

The full spring-cloud-config-server test suite was also attempted, but on Windows it encounters JGit cleanup failures where .git/objects/pack/*.pack files cannot be deleted. These failures affect unrelated JGit-based tests.

Related Issue

Fixes #2645