[Flink] Nanosecond support for Flink 1.19

[talatuyarer]

Backported from Flink 2.1 version #15475

[pvary]

Please @talatuyarer fix the PR. This should be in 1.11.0, as the main PR has already been merged, but the backport to 1.20 did not happen.

Could you please explain these differences in flink/src/main/java/org/apache/iceberg/flink/formats/avro/typeutils/AvroSchemaConverter.java:

< +          return new AtomicDataType(
< +              new TypeInformationRawType<>(false, Types.GENERIC(Object.class)));
---
> +          throw new UnsupportedOperationException(
> +              "UNION with more than 2 types is not supported in Flink 1.20 backport.");

And in flink/src/test/java/org/apache/iceberg/flink/DataGenerators.java:

< @@ -236,10 +251,12 @@ public class DataGenerators {
<  
<        genericRecord.put("date_field", DAYS_BTW_EPOC_AND_20220110);
---
> @@ -238,8 +253,10 @@ public class DataGenerators {
1798,1799c1794,1795
< -      // Although Avro logical type for timestamp fields are in micro seconds,
< -      // AvroToRowDataConverters only looks for long value in milliseconds.
---
>        // Although Avro logical type for timestamp fields are in micro seconds,
>        // AvroToRowDataConverters only looks for long value in milliseconds.
1802,1803d1797
< +      // Now that AvroToRowDataConverters correctly supports microseconds,
< +      // we must inject correct microsecond scale values into the Avro data.
1943c1937

[pvary]

Also, spotless needs to be run on the PR

[pvary]

Created a PR so we can unblock the release if we need to: #16240