Comments/docs on "bring your own table" and ALTER situation

[mieciu]

When users connect Quesma to ClickHouse and would like to work with tables which are not created by Quesma, ALTER statements doesn't work, unless attributes_* columns are present. Therefore we end up with:

1. No ALTER executed
2. Extra fields being ignored because we neither added the dedicated column nor had attributes_* column to store it.

In general, we discourage such setup in read/write scenarios - it's always better to have Quesma manage your CH tables so that it controls any required metadata.

In most cases BYOT is used in read-only, ingest-disabled situations. However, there's a simple way to mitigate this limitation:

ALTER TABLE test_db.test_table [ON cluster quesma_cluster] ADD COLUMN "attributes_values" Map(String, String)
ALTER TABLE test_db.test_table [ON cluster quesma_cluster] ADD COLUMN "attributes_metadata" Map(String, String)