Allow updating multibody constraints in the context with new specs values after Finalize()

[chen-tianjian]

Is your feature request related to a problem? Please describe.
We are attempting to model an item in a soft bag as a runtime-mutable pendulum, and we wish to leverage the multibody distance constraints. However, currently Drake does not allow updating the constraints specs after Finalize().

Describe the solution you'd like
Allow updating multibody constraints in the context with new specs values.
Currently we only care about updating distance, but we are glad to see other specs becoming mutable too.

Describe alternatives you've considered
We can also use bushing for this purpose, but (we heard that) constraints are better for numerics in SAP.

Additional context
The slack discussion is here

[sherm1]

Assigned to @amcastro-tri for disposition.