Refactor: RelayerID declared in database

[najeal]

Context and scope
RelayerID is currently declared in the database package. I think we can find a better place for the type declaration but I did not find a ideal place yet.

Discussion and alternatives
It could be moved in relayer package but it is not possible at the moment as it would create an import cycle. This would require to create some type abstraction to avoid direct dependency.
A temporary work around can be to create new package for the RelayerID but I am not a big fan.