yams #70261 [schema2sql] use named PK constraints for entity tables [deprecated]

The PK constraints for entities are not named explicitely (and use default autogenreated names by the backend). Fixing this would enable easier administration and cross db backend support.

we already have explicit names for relation tables.

syntax reminder:

CREATE TABLE ..... CONSTRAINT index_name PRIMARY KEY (col1, col2...)
CREATE TABLE tablename ( colname coltype CONSTRAINT indexname PRIMARY KEY, ...)
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