mesh bp: support embedded ghost cells for structured meshes

Codes that use structured meshes which contain ghost cells need to specify the ghost regions when publishing their data to the mesh blueprint.

One common way to do this is to specify a "real extent".

This issue proposes adding an optional "real_extent" field to the implicit topology, for example, something like this:
```
topologies/topo/coordset: “coords”
topologies/topo/type = “structured”
topologies/topo/elements/dims/{i,j,k}
topologies/topo/elements/real_exent/{imin,imax,jmin,jmax,kmin,kmax} {optional}
topologies/topo/elements/origin/{i0,j0,k0} (optional, default = {0,0,0})
```

No file or test is named in the issue. Start by locating the structured implicit-topology handling and its validation or tests, then trace how topology fields such as dims and origin are represented. Done means structured meshes can publish an optional real_extent describing ghost-cell regions without breaking existing topologies.

Written by the indexing model from the issue text.