Migration Service Improvements

New MigrationService methods

Migration getMigration(String id)

Returns a Migration object (see bewlow) given its id.

List<Migration> getMigrations()

Returns the list of all {{Migration}}s.

void run(String migrationId)

Runs a given migration: the migration is probed if needed, then is run if there is only a step matching the migration state.
If there is no step or more than one step matching the migration step, throws Exception.

Migration Object

Introduce a new Migration object to store:

• the migration id
• the migration description
• the migration description label
• the migration status (MigrationStatus object)
• the available steps

As those objects will be used in the REST API. that means creating new writers MigrationJsonWriter and MigrationListJsonWriter.