[fast-avro] FastGenericDatumReader forwards setSchema() to coldDeserializer

[krisso-rtb]

Fixes the issue described in a comment:

• Added serde customization support in Fast-Avro #520 (comment)

---

Consider below code-snippet:

FastSpecificDatumReader<SomeRecord> datumReader = new FastSpecificDatumReader<>(null, schema);
DataFileStream<SomeRecord> dataFileStream = new DataFileStream<>(inputStream, datumReader);

Writer schema passed to FastSpecificDatumReader's c-tor is null because it's unknown yet.
C-tor of DataFileStream extracts the writerSchema from inputStream and passes it to the datumReader by invoking reader.setSchema(header.schema);

This PR forwards setSchema() call to coldDeserializer which is used until the actual (fast) deserializer is generated.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b0e0a27) 46.12% compared to head (790b304) 46.12%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #534   +/-   ##
=========================================
  Coverage     46.12%   46.12%           
+ Complexity     4540     4539    -1     
=========================================
  Files           407      407           
  Lines         28423    28423           
  Branches       4637     4637           
=========================================
  Hits          13110    13110           
  Misses        13746    13746           
  Partials       1567     1567           

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