avro

AVRO-3984 [C++] Improve code generated for unions (#3047)

* AVRO-3984 [C++] Getters created by avrogencpp return a reference instead of a value to avoid calling copy constructor of large classes

* AVRO-3984 [C++] Add getter for generated unions that returns a mutable reference. This allows the user to modify values in union branches after creation (#3047)

* AVRO-3984 [C++] Add move setters for generated unions to provide a more efficient way to set a value (#3047)

* AVRO-3984 [C++] Use std::move in decode implementation of codec_traits for unions to avoid a copy (#3047)

* AVRO-3984 [C++] Generate an enum for each union type that maps the branch names to the corresponding index. This allows the user to avoid checks against "magic numbers" (#3047)

* AVRO-3984 [C++] Add additional checks for the union branch in testUnionMethods test (#3047)

* AVRO-3984 [C++] Add additional branch() method that returns the Branch enum directly, this avoids a manual static_cast (#3047)

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Co-authored-by: hwse <a35winuuvvbpa@mailbox.org>