Use PETSc CTX for SNES solver

[jorgensd]

Spawned by the discussion in https://gitlab.com/petsc/petsc/-/work_items/1883.
This is a better way to pass around the _blocks value if required (at least more PETSc like).
Maybe @dalcinl can comment on if this is better than our current approach.

[dalcinl]

Passing extra data via kargs seems reasonable. However, this approach will work as long as the SNES solver is tied to a specific instance of a problem, and not reused for another instance of problem where the kargs would not correspond. From my limited pure-human non-AI-assisted understanding of the code, things seem correct.