[Bug]: Concurrency issue when using addLogicalTypeConversions on avro model data

[RustedBones]

What happened?

Since beam 2.49 after #26320, we can see the following exception

"java.lang.NullPointerException
	at org.apache.avro.generic.GenericData.addLogicalTypeConversion(GenericData.java:116)
	at org.apache.beam.sdk.extensions.avro.schemas.utils.AvroUtils.addLogicalTypeConversions(AvroUtils.java:169)
	at org.apache.beam.sdk.extensions.avro.io.AvroDatumFactory$ReflectDatumFactory.apply(AvroDatumFactory.java:188)
	at org.apache.beam.sdk.extensions.avro.io.AvroIO$Sink.open(AvroIO.java:2126)
	at org.apache.beam.sdk.extensions.smb.FileOperations$Writer.prepareWrite(FileOperations.java:219)
	at org.apache.beam.sdk.extensions.smb.FileOperations$Writer.access$100(FileOperations.java:206)
	at org.apache.beam.sdk.extensions.smb.FileOperations.createWriter(FileOperations.java:133)

Either in the AvroIO or in the AvroCoder.
This seems to happen due to a concurrency access on the same GenericData (During initialization of the datum reader and writer, there is a high chance for avro returning the singleton instance).

The ReflectDatumFactory should avoid concurrent access to avro's model data.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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