Improve ConfigData processing code

Refactor `ConfigData` processing code to make it less awkward to
follow.

Prior to this commit the `ConfigDataLocationResolver` would take a
String location and return a `ConfigDataLocation` instance. This was
a little confusing since sometimes we would refer to `location` as the
String value, and sometimes it would be the typed instance. We also
had nowhere sensible to put the `optional:` prefix logic and we needed
to pass a `boolean` parameter to a number of methods. The recently
introduced `Orgin` support also didn't have a good home.

To solve this, `ConfigDataLocation` has been renamed to
`ConfigDataResource`. This frees up `ConfigDataLocation` to be used
as a richer `location` type that holds the String value, the `Orgin`
and provides a home for the `optional:` logic.

This commit also cleans up a few other areas of the code, including
renaming `ResourceConfigData...` to `StandardConfigData...`. It also
introduces a new exception hierarchy for `ConfigDataNotFoundExceptions`.

Closes gh-23711

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -663,7 +663,7 @@ You can use this prefix with the `spring.config.location` and `spring.config.add
 
 For example, a `spring.config.import` value of `optional:file:./myconfig.properties` allows your application to start, even if the `myconfig.properties` file is missing.
 
-If you want to ignore all `ConfigDataLocationNotFoundExceptions` and always continue to start your application, you can use the `spring.config.on-location-not-found` property.
+If you want to ignore all `ConfigDataLocationNotFoundExceptions` and always continue to start your application, you can use the `spring.config.on-not-found` property.
 Set the value to `ignore` using `SpringApplication.setDefaultProperties(...)` or with a system/environment variable.