Adding AvroToValueMapper (Spanner writer) for non-primitive types

[VardhanThigle]

TODO:

1. Handling of Maps as Json
2. Filling Array in JSON node and handling in changeEventType convert will be covered as a part b/390535288

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.58%. Comparing base (b51c933) to head (4c53d8b).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2175      +/-   ##
============================================
+ Coverage     47.05%   55.58%   +8.53%     
+ Complexity     4049     1642    -2407     
============================================
  Files           876      409     -467     
  Lines         52230    22305   -29925     
  Branches       5507     2217    -3290     
============================================
- Hits          24578    12399   -12179     
+ Misses        25892     9192   -16700     
+ Partials       1760      714    -1046     

Components	Coverage Δ
spanner-templates	70.76% <100.00%> (+1.84%)	⬆️
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	77.30% <100.00%> (+0.81%)	⬆️
spanner-live-reverse-replication	79.47% <100.00%> (+0.66%)	⬆️
spanner-bulk-migration	88.32% <100.00%> (+0.37%)	⬆️

Files with missing lines	Coverage Δ
.../cassandra/mappings/CassandraMappingsProvider.java	98.11% <100.00%> (ø)
...sandra/rowmapper/CassandraRowValueArrayMapper.java	100.00% <100.00%> (ø)
.../v2/spanner/migrations/avro/AvroToValueMapper.java	94.17% <100.00%> (+2.99%)	⬆️
...er/migrations/avro/GenericRecordTypeConvertor.java	95.73% <100.00%> (+2.08%)	⬆️

... and 485 files with indirect coverage changes

[shreyakhajanchi]

LGTM