Consider adding support for Flyway-style callbacks

[srcshelton]

Flyway supports the following callbacks, from ./sql/<callback>.${sqlMigrationSuffix:-sql} files:

• beforeMigrate
• beforeEachMigrate
• afterEachMigrate
• afterMigrate
• beforeClean
• afterClean
• beforeInfo
• afterInfo
• beforeValidate
• afterValidate
• beforeBaseline
• afterBaseline
• beforeRepair
• afterRepair

Further research is needed to determine what (if any) parameters are passed to the callback files (likely via Placeholder substitution).

[srcshelton]

Flyway supports the concept of default callbacks and custom callbacks. If callbacks are enabled that the former can be disabled causing only the latter to be executed.

[srcshelton]

It appears that - via the Java API at least - callbacks receive no additional data except for beforeEachMigrate and afterEachMigrate, which also receive a MigrationInfo object which represents a single entry in the schema_version table.

The use-case for callbacks appears to be basic "fix-up privileges" situations rather than anything more complex.

At the moment, only single-file callbacks are supported by Flyway, although specifying a directory of callbacks may be considered for future releases...