refactor: plugins

[chesedo]

This turns our service feature flags into separate crates. The long game here will be to allow vendors to write their own plugins...

TODO

• [x] Update CI

Breaking changes

This changes the main function argument attributes yet again :smiley: . These changes are as follow:

Old attribute	New attribute
shared::Postgres	shuttle_shared_db::Postgres
aws::rds::Postgres	shuttle_aws_rds::Postgres
aws::rds::MySql	shuttle_aws_rds::MySql
aws::rds::MariaDB	shuttle_aws_rds::MariaDB

The old feature flag on shuttle_service also needs to be replaced with the following crates definitions:

Old feature flag	Extra crate definition
sqlx-postgres	shuttle-shared-db = "0.5.0"
sqlx-aws-postgres	shuttle-aws-rds = { version = "0.5.0", features = ["postgres"] }
sqlx-aws-mysql	shuttle-aws-rds = { version = "0.5.0", features = ["mysql"] }
sqlx-aws-mariadb	shuttle-aws-rds = { version = "0.5.0", features = ["mariadb"] }
secrets	shuttle-secrets = "0.5.0"