Introduce shared context for migrations and add documentation

[FloThinksPi]

Previously a migration was only tested by reverting (running down migrations) until being one migration before the one that shall be tested, then executing the actual test in which all migrations run the up part even of migrations that come after the migration one wants to test.

This commit introduces the 'migration' shared context, a valuable utility for Sequel migration testing. This new module reverts the database schema to its version prior to a specific migration, provides a directory with the migration file for running a given migration in a test, and restores the schema to its full state after test completion to avoid half-migrated database scenarios. Hereby only the up part of the migration one wants to test is executed opposite to all available
migrations to construct a meaningful test case.

The accompanying updates to the documentation elaborate on the distinctive operating nature of Sequel migration tests. Also extensive documentation on Sequel migrations itself in the scope of the supported dbs and cloud_controller_ng has been added to provide a good knowledge base for writing highly sophisticated migrations that are resilient, offer high level of consistency, are compatible with mysql and psql and fast.

Constraints and restrictions on Sequel migrations and test writing have also been detailed, stressing the need to exclude Cloud Controller code in migrations and tests to maintain consistency in test outcomes and behaviours.

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[philippthun]

Here is the existing CAPI-Migration-Style-Guide. We should merge the contents and decide for a single location (and reference it from the other location).