alembic_utils dependency on alembic op in single migration

[olirice]

Execute required ops from alembic when resolving required ops for alembic_utils ReplaceableEntities

[olirice]

resolves #41

[olirice]

@mjpieters would you be able to check out this branch and test if it resolves your use-case / provide usage feedback?

[olirice]

The holdup on this is that if the alembic (non-utils) operations are expensive e.g. adding a new column with a server default to a large existing table, resolving the correct migration op for the alembic_utils entities is extremely expensive.

Currently thinking about whitelisting certain alembic ops or altering them before executing to ensure that they all execute quickly, but that is significantly more complicated that this draft

[olirice]

Seen error

ValueError: no dispatch function for object: <alembic.operations.ops.ModifyTableOps object at 0x10a4ba880>

requires investigation