Added coupler constraint

[adityapande-1995]

This PR adds a new Coupler Constraint, to address #1756.
Most of the code here is taken from MimicMotorConstraint, with changes to excite, unexcite, applyUnitImpulse and applyImpulse.

This is still a work in progress, looking for ideas on how to write a test for this, to compare MimicMotorConstraint and CouplerConstraint cc @scpeters.

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Before creating a pull request

• Document new methods and classes
• Format new code files using ClangFormat by running make format
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve all the compile warnings

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change
• Add Python bindings for new methods and classes

[scpeters]

for testing, I recommend starting with the gz-physics test (see #1756 (comment)) and then writing a custom test for dart once we have something working