Honour a wrapper's unwrap when resolving the root DataSource

[jjh75607]

DataSourceUnwrapper.unwrapRoot checks the delegate chain and the AOP target before it
looks at Wrapper.unwrap, and it stops as soon as a wrapper unwraps to itself.
unwrap(DataSource, Class, Class), right above it, does the opposite and calls
safeUnwrap first.

DecoratedDataSource from spring-boot-data-source-decorator 2.0.1 extends
DelegatingDataSource and overrides unwrap to return the HikariDataSource, but its
target is datasource-proxy's ProxyDataSource. Following the delegate chain lands on the
proxy, and a Wrapper that implements the requested interface is meant to return itself
from unwrap. The search therefore ends at the proxy and DecoratedDataSource.unwrap
never runs. DataSourceBuilder.derivedFrom is the only caller, so it derives from the proxy instead
of the pool. Nothing throws. Reported downstream as
gavlyukovskiy/spring-boot-data-source-decorator#197.

This calls unwrap first and keeps going through the delegate chain and the AOP target
when a wrapper returns itself. DelegatingDataSource and LazyConnectionDataSourceProxy
both return themselves, so the delegate chain still handles them and the lazy connection
proxy case from #50271 is unaffected. The new test fails on 4.1.x without the change.

[snicoll]

Thanks @jjh75607