Allow `${..}` property placeholders to be used in `spring.config.import`
properties. Prior to this commit, placeholders were not resolved when
binding the `ConfigDataProperty` instance. Furthermore, binding happened
too early for all placeholders to be resolved correctly. The
`ConfigDataEnvironmentContributor` class now has two states for imported
contributors, `UNBOUND_IMPORT` has the initial unbound state and is
eventually replaced with a `BOUND_IMPORT`.

Closes gh-23020