New Plugin - IMigrate

## User story

As a sysadmin
I want a consistent interface to running migrations for extensions
So that I don't have to manually run sql scripts or hunt through the code for clues.
## Feature

A new plugin for extensions to implement so that when a user runs `db upgrade PLUGIN` the migrations for the newly updated plugin get run. This could also be used for initial DB setup rather than the current approaches of:
- A paster command, named init, db-init, setup, create etc.
- Checking if the tables exist on every launch of CKAN and creating if not.

Start by reading the user story and feature description, then compare the proposed `db upgrade PLUGIN` flow with the listed paster commands and launch-time table checks. Define the extension migration interface and determine how initial setup and upgrades should be recognized as complete; no implementation files or tests are named in the issue.

Written by the indexing model from the issue text.