Annulus region masks restricted to mode='center'

Currently annulus regions (e.g. `CircleAnnulusPixelRegion`) are implemented as Compound regions and Compound region masks are restricted to use only `mode='center'` (see https://github.com/astropy/regions/blob/main/regions/core/compound.py#L68). This makes sense for generic user-defined generic regions, but not for the provided `CircleAnnulusPixelRegion`, `EllipseAnnulusPixelRegion`, and `RectangleAnnulusPixelRegion` objects for which the other `modes` are well-defined. For example, it's not currently possible to perform "exact" aperture photometry with any of these provided annulus region objects.

I thought that originally the annulus region masks could use other overlap modes (e.g., `photutils` provides this), so I suspect that this behavior changed at some point when the annulus regions were refactored. Perhaps it was unintended.

Start with regions/core/compound.py, especially the mode restriction, then inspect CircleAnnulusPixelRegion, EllipseAnnulusPixelRegion, and RectangleAnnulusPixelRegion. Confirm that the provided annulus regions support the well-defined overlap modes, including exact, so exact aperture photometry can use each one.

Written by the indexing model from the issue text.