Add support for random intialization of gaussians

[pierotofy]

This can be useful in datasets that don't have sparse points

[mikemyan]

tried to run OpenSplat on my own data using ./opensplat /path/to/my/colmap/project -n 2000. The project folder includes an image folder with all input images, a file called database.db, and the corresponding ply file. However, the error says Invalid project folder (must be either a colmap or nerfstudio project folder) .