Add equatorial stereographic projection

[Don-vip]

From #15880 we have two projections which need to implement the equatorial (0°) of the stereographic projection:

ticket:15880#comment:22

53026,?Sphere_Stereographic?,?ESRI?,53026,?Stereographic?,?PROJCS["Sphere_Stereographic",GEOGCS["GCS_Sphere",DATUM["D_Sphere",SPHEROID["Sphere",6371000.0,0.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Stereographic"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]?

54026,?World_Stereographic?,?ESRI?,54026,?Stereographic?,?PROJCS["World_Stereographic",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Stereographic"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]?

PROJ.4 implementation: ​https://github.com/OSGeo/proj.4/blob/master/src/PJ_stere.c