Move deadline_alerts into their own table for UI integration

[ferruzzi]

Moves the DeadlineAlert definitions out of the serialized_dag and into their own table in order to make UI integration easier for @bbovenzi.

I kn ow I tagged a lot of you. This is my biggest migration to date so I'd appreciate extra eyes on the SQL queries in particular. I put together a very extensive manual testing plan that consists of "paste this command into the terminal and you should see this output" steps for testing upgrading and downgrading with and without the COMPRESS_SERIALIZED_DAGS flag enabled. Double-checking that those testing commands are valid would also be a great help.

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[ramitkataria]

I just briefly looked at callback related changes but will try to take a more thorough look later if I get a chance

[Lee-W]

I took a quick look and left a few questions. will take a deeper look when I have the chance. Thanks!

[ferruzzi]

Some of those commits were trial and error so I'm cleaning up the commits and clearing ou5t some unnecessary changes.

[ephraimbuddy]

We need to be careful in updating the deadline alert through the serdag to avoid updating the wrong version since we now have dag versioning.