add support for AnyPool

[prabirshrestha]

Add support for AnyPool such that database can be chosen at runtime. match can then be used to run specific queries.

match pool.any_kind() {
    AnyKind::Postgres => {...},
    AnyKind::Sqlite => {...},
    AnyKind::MySql => {...},
}

[casbin-bot]

@hackerchai @PsiACE

[prabirshrestha]

In the meantime I have file an issue on Sqlx side for making it easy to convert to and from AnyPool. launchbadge/sqlx#2162. This could help every other projects.

[hsluoyz]

@prabirshrestha can you provide more details about this requirement?