Globally constrained current flow on closed toroidal domain

[alexanderianblair]

Adds an MFEM example enforcing a global (topological) constraint on the loop voltage of a closed toroidal conductor to drive a non-trivial current flow. Closes #31404.

Solution originally based on the method detailed here, as implemented in the MFEM-based library hephaestus, but with a number of fixes to the hephaestus implementation for adding transition regions in parallel and from multiple domains.

Adds a new SubMesh class to specify the set of elements adjacent to a planar cut surface, used to define the (constrained) source field.

Also adds a number of auxiliary AuxKernels and ICs as required to impose the constraint and for testing:

• MFEMSumAux for storing the sum of two MFEMVariables, used to combine the external and internal fields here
• MFEMScalarBoundaryIC, for specifying an initial condition on a scalar variable on a boundary, used here set the constrained values of an auxiliary source potential on the cut plane
• MFEMMixedGradGradKernel and MFEMMixedVectorMassKernel, to add MixedBilinearFormIntegrators similar to the diffusion and vector mass integrators, but for cases where the test and trial variable may differ - used here for adding source terms to the weak form
• MFEMVectorFEInnerProductIntegralPostprocessor, to report the integral of the dot product of two vector MFEM variables, used here to report the power dissipated by the coil as a test metric.

Additional documentation has also been added to the MFEM examples index detailing the method implemented in the new test.

[moosebuild]

Job Coverage, step Generate coverage on 292dcef wanted to post the following:

Framework coverage

	fef103	#31405 292dce
	Total	Total	+/-	New
Rate	85.66%	85.68%	+0.02%	96.38%
Hits	118023	118279	+256	266
Misses	19760	19770	+10	10

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This comment will be updated on new commits.

[moosebuild]

Job Precheck, step Clang format on 1ba016a wanted to post the following:

Your code requires style changes.

A patch was auto generated and copied here
You can directly apply the patch by running, in the top level of your repository:

curl -s https://mooseframework.inl.gov/docs/PRs/31405/clang_format/style.patch | git apply -v

Alternatively, with your repository up to date and in the top level of your repository:

git clang-format 1f9d319c30b1f731e4e7890484c3a82114b17427