Make the database kind queryable from AnyConnectionKind and AnyConnection

[05storm26]

No description provided.

[abonander]

We already merged a PR to add a way to get the AnyKind of an AnyPool, it just hasn't been released yet: https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/pool/mod.rs#L348

I don't see the point of replacing the AnyPool typedef with a custom type, that could actually be a breaking change.

Adding kind() to AnyConnection seems like a good improvement though.

[05storm26]

@abonander Oh, I have missed that. (I have tried to do that same thing using just impl AnyPool for implementing a getter for AnyKind initially but didn't realize that SharedPool has the connect_options that can be used to get the actual AnyKind. So I rewrote AnyPool to be able to save the AnyKind from the connect_options during the initialization/creation. But yes it is unnecessary.)

I removed the rewrite of AnyPool and just left the addition of kind() to AnyConnectionKind and AnyConnection.

[05storm26]

I have also added a kind getter to PoolConnection<crate::any::Any>