Add helpers for common add/remove setting migrations

[tjkirch]

I'm still thinking about how we could automate away the need for common migrations, but this at least makes adding or removing a setting a one-liner that you barely have to think about, rather than copy/paste or new code each time.

Testing done:

Rebuild and ran the remove-region migration forward and backward. It has no work to do on upgrade:

$ cargo run -- --log-level trace --datastore-path datastore-sample/current --migration-directories ./migration-binaries --migrate-to-version 0.2
...
19:59:08 [DEBUG] (1) migrator: Sorted migrations: ["migrate_v0.2_remove-region"]
19:59:08 [ INFO] New data store is being built at work location /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2_AqZorFmcnAhXpUNF
19:59:08 [ INFO] Running migration command: "./migration-binaries/migrate_v0.2_remove-region" "--forward" "--source-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1_j52ioMoTEFBcHM0Z" "--target-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2_AqZorFmcnAhXpUNF"
19:59:08 [DEBUG] (1) migrator: Migration stdout: AddSettingMigration(settings.aws.region) has no work to do on upgrade.
AddSettingMigration(settings.aws.region) has no work to do on upgrade.

19:59:08 [DEBUG] (1) migrator: Migration stderr: 
19:59:08 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2 to point to v0.2_AqZorFmcnAhXpUNF
19:59:08 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0 to point to v0.2

On downgrade it did its job:

$ cargo run -- --log-level trace --datastore-path datastore-sample/current --migration-directories ./migration-binaries --migrate-to-version 0.1
...
19:59:23 [DEBUG] (1) migrator: Sorted migrations: ["migrate_v0.2_remove-region"]
19:59:23 [ INFO] New data store is being built at work location /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1_lm9HzZmqysxr94CJ
19:59:23 [ INFO] Running migration command: "./migration-binaries/migrate_v0.2_remove-region" "--backward" "--source-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.2_AqZorFmcnAhXpUNF" "--target-datastore" "/home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1_lm9HzZmqysxr94CJ"
19:59:23 [DEBUG] (1) migrator: Migration stdout: Removed settings.aws.region, which was set to '"us-west-2"'
Found no settings.aws.region to remove

19:59:23 [DEBUG] (1) migrator: Migration stderr: 
19:59:23 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0.1 to point to v0.1_lm9HzZmqysxr94CJ
19:59:23 [ INFO] Flipping /home/ANT.AMAZON.COM/tjk/work/thar/workspaces/api/migration/migrator/datastore-sample/v0 to point to v0.1

[etungsten]

🦆

[tjkirch]

This push was just a rebase onto changes from #644.

[tjkirch]

This push was just a rebase onto changes from #644.

[tjkirch]

Just a rebase on develop.