Move responsability to add required headers from clients to the transport client

[lilgallon]

this is not really part of the scope of this PR, but adding those parameters to the withRequiredHeaders makes it more obvious: it's tedious and error prone to have all the clients call this method to add the headers. I think we should move the responsibility to the transportClient to add the required headers in the send method. But I tihnk we can move forward with this PR and address this point in another PR, as well as the review of how authentication and tokens are used / named.

Originally posted by @xhanin in #14 (comment)

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It concerns

• .withRequiredHeader()
• .authenticate()

[atschabu]

As this requires an object holding a reference to the PartnerRepo and partnerUrl I can see two simple ways of making this work.

a) making TransportClient abstract, and implement send to add these things before calling a doSend that needs to be implemented by actual implementation. Migration effort would be small, and extensible, as implementation could still override existing methods

b) have a "middleware" that implements send method and wraps the actual transport client

[lilgallon]

The a) approach would be nice. It's important to be extensible on this part