[P4Testgen] Open up P4Testgen interface.

[fruffy-bfn]

• Add PRIVATE/PUBLIC labels to CMake library dependencies in the P4Tools framework.
• Remove getPortNumWidthBits and replace it with the constants in the respective back ends.
• Remove the seed as parameter from the back end implementation. That should be inferred from the options class.
• Make FinalState a movable class by replacingconst Model & with std::reference_wrapper<const Model>.
• Introduce a helper function in the testgen.cpp class.

@vlstill This PR makes some API changes, test backends will not have a seed argument and instead grab it from the static options class and getPortNumWidthBits is replaced by referencing a constant instead.

[jnfoster]

A quick review, but looks good!

[jnfoster]

😢