Resolve "Parking motion planner"

This adds a package with a parking motion planner library. I'm opening this as one large MR instead of starting with the interface stubs because I'll be away for two weeks so there is plenty of time for people to have a first look. While I won't be able to deal with any changes for that time, my hope is that having the code in the open already helps people understand conceptually what it does so work on the planning front can continue more efficiently.

The main interface to the planner is defined in include/parking_planner.hpp, so I'd start reading there and in the implementation thereof in src/parking_planner.cpp to get an overview.

There are a variety of tests in the test folder. Of particular interest are the "integration" tests (integration in the sense of combining the different parts of the planner - might have to rename those here because for the purposes of Autoware they are unit tests) in test_integration.cpp. There, the external interface is called for a variety of situations.

@JWhitleyWork suggested that he could do the code style and convention review and @christopher.ho could review functionality-wise, so I'm assigning the MR accordingly.

Closes #186 (closed).