Pre-compile svg icon assets using vector_graphics_compiler

[aebrahim]

The social login icons are packaged as strings containing the raw svg, which are then rendered using flutter_svg. However, under the hood, using flutter_svg means also including its dependency vector_graphics_compiler with the application, which at runtime converts raw svg to a compiled VectorGraphic to display.

Given that the svg string is known at build time, it is possible to pre-compile it as a VectorGraphic at build time. Advantages:

1. Would no longer pull in the vector_graphics_compiler dependency, which adds ~180 KB to the build.
2. Slightly more efficient runtime as the precompiled VectorGraphic can be a const.

[lesnitsky]

Great suggestion, I will be looking into implementing this. Will be tracked under #247