mbp: Should consider implementing "MultibodyPlant subgraph"

[EricCousineau-TRI]

Is your feature request related to a problem? Please describe.
See issues #12203, #13177, and #13074, and related issues.

AFAIK, Drake at present does not have a great affordance for decomposing / (re)composing multibody plants, e.g. for making control models, caching parsing of models, etc.

This issue presents a potential solution for possibly all of these things.

Describe the solution you'd like
Add the ability to select subsets of a MultibodyPlant and (optionally) corresponding SceneGraph components, remove portions based on their dependencies, then copy those portions (and their corresponding default and non-default state) to a new (MultibodyPlant, [optional] SceneGraph) pair, providing the mapping.

See prototype:
repro/.../multibody_plant_prototypes, MultibodyPlant Subgraph (permalink)
Please see:

• Docstring for multibody_plant_subgraph for more details.
• Unittest for example workflows.

IMO, a broader systems equivalent is out of scope.

Describe alternatives you've considered
Those described in aforementioned issues.

Additional context
N/A

\cc @azeey