Use Retrofit in YoutubeParsingHelper.

[Isira-Seneviratne]

• [x] I carefully read the contribution guidelines and agree to them.
• [x] I have tested the API against NewPipe.
• [x] I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.

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Use Retrofit to handle making the requests to the supported services' APIs and parsing the responses.

Note:

• I have not been able to test YouTube Music with NewPipe as it is not available for me.
• I have set the Retrofit and OkHttp versions to 2.6.4 and 3.12.13 to support Android versions 4.0 - 4.4, as newer versions of those libraries require at least Android 5.0 due to relying on TLS 1.2.