fix: Check for snowflake functions when setting up materialization engine

[benchoncy]

What this PR does / why we need it:

When running feast apply using the snowflake materialization engine, feast only checks if the stage in snowflake exists and assumes the functions have been created. This allows a case where if anything goes wrong during function creation, such as permission issues, a stage can be created while the functions are not. Following attempts to feast apply see the created stage and tell the user that the functions exist without creating them. This leads to materialization on snowflake to fail down the line due to missing functions.

This PR aims to change the apply logic to instead check for snowflake functions before deciding if action is required.

Which issue(s) this PR fixes:

Misc

A current workaround for this is to delete the stage in Snowflake before running the next feast apply.

[tokoko]

Why remove the existing stage check? Does checking for functions make checking for stage redundant?

[benchoncy]

Why remove the existing stage check? Does checking for functions make checking for stage redundant?

Thanks for the review, @tokoko. My understanding is the stage is a stepping stone to creating the functions as a place to upload and store files required by the functions. The main objective is to create the functions, as part of that feast creates a stage, but outside of these functions, this stage goes unused.

The functions cannot exist without the stage, and the stage is not reused elsewhere, so a stage check is redundant when checking for functions. Instead, I've added IF NOT EXISTS to the stage creation to make the operation idempotent and avoid failure if the stage already exists. I hope this better explains my reasoning.

[tokoko]

thanks, makes sense. I guess this still doesn't account for the changes in function definitions themselves, right? If we add some function to be deployed, a user will have to remove the stage or all functions for them to be correctly recreated. A problem for another day...

[tokoko]

lgtm