Feat: Implement create_table_if_not_exists

[hussein-awala]

closes: #406

This PR implements the create_table_if_not_exists method to create the table if it doesn't exist, and return the existing table if it exists instead of raising a TableAlreadyExistsError exception.

[Fokko]

I think we should have some more discussion around this. I would prefer to have, analog to SQL, a CREATE OR REPLACE API.

[sungwy]

I think we should have some more discussion around this. I would prefer to have, analog to SQL, a CREATE OR REPLACE API.

This feels analogous to the SQL CREATE TABLE IF NOT EXISTS, which allows applications to run the statement idempotently in a Production deployed code without throwing an exception. Hence I'd argue it fills in a different niche than CREATE OR REPLACE, which effectively replaces an existing table (which is exactly what we want to avoid doing).

[hussein-awala]

Exactly, this implements CREATE TABLE IF NOT EXISTS and not CREATE OR REPLACE.

If you prefer to have a new method create_table_if_not_exists, I can move the table creation block to a new private/protected method, and use it in create_table and the new create_table_if_not_exists.

[Fokko]

@hussein-awala Thanks! You're right, and I think create_table_if_not_exists would be a cleaner solution, see #406.

[hussein-awala]

I will update the PR then 👍

[hussein-awala]

I think it's ready 🤞

[Fokko]

Nice! It would have been easier to write an integration test, but great job on the mocking 👍

[Fokko]

@hussein-awala This looks great, thanks!