[Backend API] Azure Key Vault for OpenAI API keys

[justinyoo]

AS a backend API app
I WANT to retrieve the list of Azure OpenAI API keys
SO THAT I can use them as configuration variables

[justinyoo]

1. 빌더 패턴 설계
2. appsettings.json 읽어들일 수 있도록 구현
3. Azure Key Vault 사용할 수 있도록 구현

[justinyoo]

1. [AppHost] Add bicep for Key Vault #193
2. [Backend API] Add Key Vault client #195
3. [AppHost] Add extra orchestration #165
4. [Backend API] Builder pattern that returns OpenAISettings #166
5. [Backend API] OpenAISettingsBuilder.WithAppSettings() 구현 #167
6. [Backend API] OpenAISettingsBuilder.WithKeyVault() 구현 #168
7. [Backend API] Add extension method AddOpenAISettingsFromKeyVault #164