Implement broadcast join

[nirandaperera]

Implement broadcast join. This will be important where one relation is significantly smaller than the other. Then, it is efficient to broadcast/ all-gather the smaller table rather than shuffling.
All the required building blocks are already available. Should support all join variations (left, right, inner, outer).

Steps:

1. Broadcast/ all-gather right table (this should be the small one)
2. Do a local join

[chathurawidanage]

Note: In a left join, you can only broadcast the right table. In a right join, you can only broadcast the left table regardless of the smallest table.

[nirandaperera]

Note: In a left join, you can only broadcast the right table. In a right join, you can only broadcast the left table regardless of the smallest table.

Precisely!