firebase_ui_oauth: Pre-compile svg icon assets using `vector_graphics_compiler`

[lesnitsky]

Discussed in https://github.com/firebase/FirebaseUI-Flutter/discussions/240

^{Originally posted by **aebrahim** January 4, 2024} 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.

[github-actions[bot]]

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[aebrahim]

This issue still requires attention - flutter_svg is still listed as a dependency at the current version here: https://github.com/firebase/FirebaseUI-Flutter/blob/93bfe605d9b45fe14c2b7a37567618d88e7f49b2/packages/firebase_ui_oauth/pubspec.yaml