[Feature Request]: BQ: Allow map values to be nullable in GCP SDK BigQueryIO

[assaadhjb]

What would you like to happen?

in this line :

beam/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryAvroUtils.java

Line 577 in 6043660

.setMode("REQUIRED");

We enforce MAP types to have required values, this is a huge setback, especially that in previous versions it was not required, for users who would like to produce messages with optional values in their MAP types.
PR where it was added: #33389

Issue Priority

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

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