Configure Proguard for mirai-core-all

[Him188]

close #990

[Him188]

@AdoptOSS Thanks!

[zhaodice]

经测试ktor和netty、bouncycastle经过如下混淆依然可以使得mirai正常运行，体积（apk）减少了1MB，虽然只是粗浅测试，但收发消息、重连什么的似乎没问题？（测试环境 2.7.1-dev-1）

# ktor
-keep class io.ktor.utils.io.pool.DefaultPool { *; }

#netty
-keepattributes Signature,InnerClasses
-keep class io.netty.bootstrap.Bootstrap { *;}
-keep class io.netty.buffer.AbstractByteBufAllocator { *;}
-keep class io.netty.util.ReferenceCountUtil { *; }
-dontwarn io.netty.**
-dontwarn sun.**

#bouncycastle
-keep class org.bouncycastle.jce.provider.**{ *;}
-keep class org.bouncycastle.jcajce.provider.asymmetric.** {*;}
-keep class org.bouncycastle.jcajce.provider.keystore.** {*;}