leverage ServiceConnector to setup connection to keyvault and DB - Keyvault reference

[yungezz]

leverage ServiceConnector to setup connections to keyvault and DB.
Use AppService Keyvault reference feature to refer secrets in Keyvault, instead of direct access keyvault.

• both local debug and cloud experience work
• Keyvault reference in Appservice feature provides more security in cloud.
• customize output name(appsetting names) like sample https://github.com/Azure-Samples/todo-csharp-sql/pull/1/files#diff-c49282ec82c441d0ca28744efdbdae6288715e320eb90772ac149c9096f7eeedR118-R120