Add embedded boundaries in the electrostatic solver

[RemiLehe]

As discussed off-line, it would be nice to add embedded conducting boundaries for electrostatic solver in WarpX.
This issue summarizes some of the points that were discussed:

• We would probably use the new embedded boundary feature, implemented here: Start Embedded Boundary development #1641
• We would then use AMReX's linear solvers with embedded boundaries when solving the Poisson equation
• This might require to place the rho and phi fields at the cell-centers instead of the nodes. As a consequence, the calculation of the gradient of phi (to obtain E) might need to be modified.
• We could then add an automated test, e.g. with KV beam in a cylindrical pipe: see page 31 of this document

[WeiqunZhang]

An example of AMReX's EB linear solver is here https://github.com/AMReX-Codes/amrex/tree/development/Tests/LinearSolvers/CellEB.

[dpgrote]

This has been implemented in PR #2067 and this issue can be closed.