AVRO-2954 Make JAVA org.apache.avro.generic.GenericContainer Serializable

[Karl-WangSK]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Avro Jira issues and references them in the PR title. For example, "AVRO-2954: My Avro PR"
  • https://issues.apache.org/jira/browse/AVRO-2954
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

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  4. Subject uses the imperative mood ("add", not "adding")
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Documentation

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  • All the public functions and the classes in the PR contain Javadoc that explain what it does

[Karl-WangSK]

surprise. test failed , it has nothing to do with this pr tho