Update Maliput API with new types

[Agustin Alba Chicar]

LanePositionAPI was moved to stop being a structure but to hold an Eigen Vector and start being a class. This affects the code inside Lane as we use it all over the private API.

This is a first change as part of a bigger one. The parent issue is:

• https://github.com/RobotLocomotion/drake/issues/4542

Current child PRs:

• https://github.com/RobotLocomotion/drake/pull/5997
• https://github.com/RobotLocomotion/drake/pull/6009
• https://github.com/RobotLocomotion/drake/pull/6062

[Agustin Alba Chicar]

Changed code to pass the tests for the changes introduced in #5997. #6009 is not merged yet.

[Agustin Alba Chicar]

New PR has been updated --> when it's closed we need to update our code with it.

[Agustin Alba Chicar]

This was set on-hold so we are going to move on the implementation. If there is a new change on the API, we will raise an issue to solve it.