[Feature Request] Add implicit coercion modes between types

[alancai98]

PLK is currently implemented to strictly follow the SQL99 spec, which does not define implicit type coercions between type families (e.g. comparing an int with a varchar without any casting). partiql/partiql-lang#97 clarifies that this is an implementation-defined behavior. We should consider making the implicit coercions configurable:

• set of implicit coercion modes -- coercion_postgresql, coercion_sql, coercion_redshift or
• possibly some catalog level ability to define CASTs and make certain implicit like PostgreSQL's CREATE CAST -- https://www.postgresql.org/docs/current/sql-createcast.html