Create tables for opportunity attachments

[acouch]

We want to create the following two tables in our SQLAlchemy models:

Only the created_by, updated_by, and legacy_folder_id columns should be nullable. Note you can use the python typing to dictate this. my_field: Mapped[int] is not nullable. my_field: Mapped[int | None] is nullable.

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The lk_opportunity_attachment_type_id lookup table should follow our conventions for setting up a lookup table. We will only have two values at the moment for this lookup enum:

• notice_of_funding_opportunity
• other

See: https://github.com/navapbc/simpler-grants-gov/blob/main/documentation/api/lookup-values.md for further details on how this works.

Acceptance criteria

• Tables created in our SQLAlchemy models
• Migrations created via Alembic
• Lookup table values configured (should auto-populate their table when running migrations)