Fix for Key Vault reading in AzureAppConfigurationBuilder (#230)

[Juraj2]

• Use GetCredential() for Key Vault readings and Add overload for SecretClientOptions in AzureAppConfigurationBuilder
• This is fixing AzureAppConfigurationBuilder to read App Configuration Key-value references to Key Vault in the same way as AzureKeyVaultConfigBuilder

Addresses #230

[Juraj2]

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