clickhouse_loader.py : support schema overrides in yaml

[aadant]

We should be able to override the schema using a yaml file

• data type mapping

• default table settings

• default type conversion

  • choice between LowCardinality versus String
  • choice how to convert binary types hex / base64

• custom table settings per regex / table

  • sort key
  • column data type
  • partitioning key
  • table settings
  • table engine

• filtering out a table from migration / replication

  • include via list and regex
  • exclude via list and regex