Implement support for slack variables on edges in GraphOfConvexSets

[bernhardpg]

This addresses #21003.

The unit tests implement the two examples from the issue above.

Note 1: I named the function NewSlackVariables. It could alternatively be called NewContinuousSlackVariables to be closer to the naming conventions of the member functions in MathematicalProgram.

Note 2: I could also add another helper function called NewSlackVariable (or NewContinuousSlackVariable) (singular, in addition to plural) if we think that would be natural. I imagine that for most use-cases, most people will only add one slack variable at a time.

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This change is 

[bernhardpg]

@TobiaMarcucci FYI

[sherm1]

+@RussTedrake for feature review, please (or delegate)

Reviewable status: LGTM missing from assignee RussTedrake(platform), needs at least two assigned reviewers, commits need curation (https://drake.mit.edu/reviewable.html#curated-commits), missing label for release notes (waiting on @bernhardpg)

[RussTedrake]

Reviewed 2 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee RussTedrake(platform), needs at least two assigned reviewers, commits need curation (https://drake.mit.edu/reviewable.html#curated-commits), missing label for release notes (waiting on @bernhardpg)

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geometry/optimization/graph_of_convex_sets.cc line 959 at r2 (raw file):

    prog.AddDecisionVariables(e->z_);
    prog.AddDecisionVariables(e->ell_);
    prog.AddDecisionVariables(e->slacks_);

we need to add this to SolveConvexRestriction, too.

[RussTedrake]

One question, I guess, is whether we want to implement the "vertex isa MathematicalProgram" idea first, and just give more direct access to MathematicalProgram's API instead of introducing this new method?

Reviewable status: 1 unresolved discussion, LGTM missing from assignee RussTedrake(platform), needs at least two assigned reviewers, commits need curation (https://drake.mit.edu/reviewable.html#curated-commits), missing label for release notes (waiting on @bernhardpg)