Feature flag: Implement tasks version checking

[KonstantinTyukalov]

Type: Feature

Related issue: microsoft/azure-pipelines-vscode#206

Description:

The Azure-pipelines vscode extension requires to support the feature flag provided from the config, that stored in the settings.json vscode user config.

This should activate the feature which will search the last versions of the tasks from the tasks schema file by enums matching

This flag will be sent from the client to the server and should be handled

Possible solution:

• Add support of the flags provided by the client. As I know, it should be realized at the server and service both
• Create a matching function which will search the last versions of the tasks from schema enums. If the current task didn't in the last versions list, add the alert like here with providing the new task version