Add support for geometry fields to memory provider

[nyalldawson]

Since the postgres provider supports these, the memory provider MUST also support them or we risk data loss

Temporarily includes #51842, #51844, #51845

[elpaso]

LGTM, I'm a little concerned about if (and possibly how) store the CRS of the geometry. That piece of information is not available in plain geometry (referenced has it).

Did you consider that?

[nyalldawson]

LGTM, I'm a little concerned about if (and possibly how) store the CRS of the geometry. That piece of information is not available in plain geometry (referenced has it).

Did you consider that?

Well, the memory provider will accept referenced geometry values happily here. So we will gain support for eg reading values from postgres, storing in intermediate memory layers, and then writing back out to another postgres layer without any loss.

The issue comes in when features are edited and we force display of geometry values in a text widget. That situation isn't improved here. I still think that the correct approach there is a dedicated widget for geometry values in attribute forms, something like nyalldawson@3343750