Add support to new Youtrack REST API as legacy API will be deprecated

[mkhadher]

Youtrack deprecate old legacy which is using /rest endpoint api and xml-based starting from July 2021. This library use legacy youtrack API and will not be useful when Youtrack deprecate legacy API starting from July 2021

Full details https://blog.jetbrains.com/youtrack/2021/02/discontinuing-the-legacy-rest-api-action-required/

[allburov]

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