[YouTube] Add utility class to handle player requests fetching

This internal class, YoutubeStreamHelper, has to be public in order to
be used by subpackages of the service's root one.

It supports poTokens, HTML5 signatureTimestamp property, embed context
and multiple InnerTube clients. It is meant to replace the
corresponding methods in YoutubeParsingHelper, in order to reduce the
class' size, code duplicates and improve its readability.

It adds a new way to get age-restricted videos' streams, the only ones
which are playable in YouTube embeds, which is not very common.

Author: AudricV