Add dual solution to OsqpSolverDetails.

[hongkai-dai]

cc @pangtao22

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This change is 

[hongkai-dai]

+@EricCousineau-TRI for both reviews please, thanks!

Reviewable status: LGTM missing from assignee EricCousineau-TRI(platform), needs at least two assigned reviewers (waiting on @EricCousineau-TRI)

[EricCousineau-TRI]

+(status: single reviewer ok)

Reviewed 5 of 5 files at r1.
Reviewable status: 1 unresolved discussion (waiting on @hongkai-dai)

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solvers/osqp_solver.h, line 33 at r1 (raw file):

  // y contains the solution for the Lagrangian multiplier associated with
  // l <= Ax <= u. The Lagrangian multiplier is set only when OSQP solves
  // the problem. Notice that the order of the linear constraints are linear

nit Can you make this a docstring? ///

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solvers/test/osqp_solver_test.cc, line 130 at r1 (raw file):

    EXPECT_EQ(result.get_solver_details<OsqpSolver>().status_val, OSQP_SOLVED);
    EXPECT_TRUE(CompareMatrices(result.get_solver_details<OsqpSolver>().y,
                                Eigen::Vector3d(0, 0, -0.0619621), 1E-4));

BTW This tolerance seems high?

[hongkai-dai]

Reviewable status: complete! all discussions resolved, LGTM from assignee EricCousineau-TRI(platform)

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solvers/test/osqp_solver_test.cc, line 130 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

BTW This tolerance seems high?

Done. OSQP is not very accurate.

[EricCousineau-TRI]

Reviewed 2 of 2 files at r2.
Reviewable status: commits need curation (https://drake.mit.edu/reviewable.html#curated-commits)