fix(proguard): keep enum members classes

[iurysza]

Summary

This PR addresses issue #29, which caused consumer apps to crash when using the library in release mode.

Issue Details

Without this rule you get this when the consumer app has Proguard/R8 minification enabled:

Caused by: java.lang.NoSuchFieldException: UNKNOWN
  at java.lang.Class.getField(Class.java:2337)
  at com.google.ai.client.generativeai.internal.util.FirstOrdinalSerializer.deserialize(SourceFile:2)
  at com.google.ai.client.generativeai.internal.api.server.FinishReasonSerializer.deserialize

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[iurysza]

Sry, I guess this was already addressed in #23