Add the ability to store IHardwareRepresentation (HAST-243)

[Piedone]

This would make it possible to generate the hardware on one machine and utilize it on another one. So, basically caching the whole hardware representation locally.

Now caching is done on the hardware description level only and you need to be only for the GenerateHardware() call anyway. This would also make it possible to use Hastlayer offline like it is necessary for embedded systems.

Jira issue

[Piedone]

This is now covered for Vitis. Needs to test if GenerateHardware() can be completely decoupled from the GenerateProxy() method call, e.g. for offline use (it can be invoked but shouldn't do VHDL generation and hardware compilation).