Add meas_extensions_convolved to lsst_distrib

meas_extensions_convolved is a plugin for the meas_base measurement framework. It convolves images before measuring aperture fluxes. The convolution provides a rough match to the PSF in other observations, and so aperture measurements on the convolved image are useful for:

1. Measuring the flux that would be obtained with a fiber spectrograph, by using an aperture equal to the size of the spectrograph fiber and measuring with multiple target seeings to allow interpolation of the flux at the seeing of the spectroscopic observation.
2. Measuring the color of galaxies in variable seeing, by using a common aperture and target seeing in each band.

This functionality is actively being used by the DRP team.

I propose to add this package as a dependency of lsst_distrib so that it can benefit from regular CI and so that it is distributed with future stack releases.