Generalize schema retriever

[sahithi03]

Changes made according to the Github issue - #245

• Added regexRouter SMT which replaces the topic field of each SinkRecord with (dataset)=(tablename)
• Removed the topicToTableResolver class since it will no longer be used. Refactored and adjusted behavior accordingly to make use of sinkRecord with topic field containing destination table.
• SchemaRetriever API changed to retrieve the schema from each SinkRecord.
• Removed SchemaRegistrySchemaRetriever and MemorySchemaRetriever as these retrieve schema based on the topic and wouldn't be compatible with the new API and SMTs.
• Implemented Schema Unionization logic to retrieve the schema from a batch of sink records with heterogenous schemas and for Schema Updates - Currently handles only two cases:
  • Adding new fields to the schema
  • Changing a field's mode from Required to Nullable.
• Updated Java doc
• Integration tests needed some changes to work with SMTs and schema unionization.
• Unit tests yet to be fixed.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #291 into generalization-feature will decrease coverage by 4.76%. The diff coverage is 45.68%.

@@                     Coverage Diff                      @@
##             generalization-feature     #291      +/-   ##
============================================================
- Coverage                     70.87%   66.10%   -4.77%     
+ Complexity                      301      267      -34     
============================================================
  Files                            32       32              
  Lines                          1538     1484      -54     
  Branches                        164      152      -12     
============================================================
- Hits                           1090      981     -109     
- Misses                          390      450      +60     
+ Partials                         58       53       -5     

Impacted Files	Coverage Δ	Complexity Δ
.../kafka/connect/bigquery/BigQuerySinkConnector.java	78.37% <ø> (+10.45%)	11.00 <0.00> (-3.00)	:arrow_up:
...ect/bigquery/retrieve/IdentitySchemaRetriever.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...om/wepay/kafka/connect/bigquery/SchemaManager.java	28.28% <4.28%> (-48.19%)	6.00 <0.00> (-3.00)
...afka/connect/bigquery/write/row/GCSToBQWriter.java	80.00% <25.00%> (ø)	13.00 <1.00> (ø)
...ect/bigquery/write/row/AdaptiveBigQueryWriter.java	39.68% <40.00%> (-0.95%)	6.00 <1.00> (ø)
...wepay/kafka/connect/bigquery/BigQuerySinkTask.java	57.21% <48.57%> (-1.74%)	27.00 <7.00> (ø)
...ka/connect/bigquery/utils/SinkRecordConverter.java	66.66% <66.66%> (ø)	3.00 <3.00> (?)
...onnect/bigquery/config/BigQuerySinkTaskConfig.java	95.38% <75.00%> (-0.21%)	14.00 <0.00> (ø)
...ka/connect/bigquery/config/BigQuerySinkConfig.java	78.94% <90.47%> (-2.83%)	16.00 <1.00> (-13.00)
...nect/bigquery/write/batch/GCSBatchTableWriter.java	86.20% <100.00%> (-0.46%)	3.00 <2.00> (ø)
... and 5 more