feat: make appflowy-cloud migrate use feature flags

[Horusiath]

This PR modifies how we're running appflowy migrations. It basically provides a custom migration source capable of providing alternative migration sub-directories to be included/excluded in migration process. These can be used for conditional features like AI search/indexing and are execute in their order to creation.

It only affects migrations run by appflowy-cloud start process, and NOT by sqlx migrate CLI command. To do that we would need to send PR to sqlx itself.

It also introduces an ai feature flag that can is used for conditional inclusion of search feature. (probably will need to change the name to something like vector-search).