Introduce `classpath*:` support for `ResourceLoader#getResource` with fully specified resource path

[jhoeller]

Inspired by #36292, generalizing classpath*: support for a fully specified resource path without pattern resolution:

Non-pattern classpath*: support could be built into (Default)ResourceLoader, so that a getResource("classpath*:messages.properties") call does not fail like right now but rather expose a merged Resource handle with InputStream access to all same-named resources on the classpath.

This implies moving the CLASSPATH_ALL_URL_PREFIX constant to the top-level ResourceLoader interface with corresponding behavior for fully specified URL locations in a plain getResource call - next to the existing pattern-aware semantics for getResources in ResourcePatternResolver which would remain unchanged.