A mechanism for defining migrations to perform when changing the MDES version of a deployment, plus the migration for 3.0 to 3.2

Defines `Mdes::VersionMigrator`, a utility which has a list of possible MDES version migrations and which can find the correct series to go from the current version to a requested version.

Also includes the migration for MDES 3.0 to 3.2. This is an example of the simplest possible migration — it has no semantic changes to existing value elements. A generic migration implementation is provided for situations like this one. An instance of that generic migration is registered as applying to a 3.0 to 3.2 conversion.