Add a parameter to join to upcast join columns

[ddutt]

Problem description

In many situations, owing to various disparate processing, storing etc, different tables have the same field stored as a slightly different type (i16 vs i32 or i32 vs i64 etc.). The fields and their possible types isn't always possible to identify and fix upfront due to various reasons.

Adding a key such as keys_to_supertypes to the join function, to upcast the join column datatypes would be most useful in solving this issue.

Opening a new issue as per discussion with @ritchie46