ci: run size comparisons for iOS/Android release artifacts on PRs

At the moment, the [coverage job only compares size differences with the core artifact](https://github.com/lyft/envoy-mobile/blob/f50aaa57033decf68d2e961f7716ecb9a5f42402/.github/workflows/perf.yml#L25). Adding jobs to additionally do size comparisons of the iOS/Android artifacts would help us catch issues like https://github.com/lyft/envoy-mobile/pull/864 where Bazel rules or compilation logic changes on only one platform.