Refactor `BootstrapRegistry` support following initial prototype work
with the Spring Cloud team.

This update splits the `BootstrapRegistry` API into `BootstrapRegistry`,
`BootstrapContext` and  `ConfigurableBootstrapContext` interfaces and
moves it to the same package as `SpringApplication`.

A new `Bootstrapper` interface has been introduced that can be added
to the `SpringApplication` to customize the `BootstrapRegistry` before
it's used.

Closes gh-23326