Add support for stored procedures

[rod-glover]

Define ReplaceableProcedure and supporting SQLA components, parallel to ReplaceableFunction to create stored procedures.

See https://www.postgresql.org/docs/current/sql-createprocedure.html

Check whether SQLA / Alembic has added support for these now. May be able to simplify both this task and our codebase if so.

[rod-glover]

https://github.com/olirice/alembic_utils now has support for functions (and several other desirable things), but not for procedures. However, it might be better going forward to use alembic-utils for all these things, and consider cloning its PGFunction tooling for procedures.