Prior to this commit, the Spring Framework build would be using the
propdeps Gradle plugin to introduce two new configurations to the build:
"optional" and "provided". This would also configure related conventions
for IDEs, adding those configurations to published POMs.

This commit removes the need for this plugin and creates instead a
custom plugin for an "optional" configuration. While the Eclipse IDE
support is still supported, there is no need for specific conventions
for IntelliJ IDEA anymore.
This new plugin does not introduce the "provided" scope, as
"compileOnly" and "testCompileOnly" are here for that.

Also as of this commit, optional/provided dependencies are not published
with the Spring Framework modules POMs annymore.
Generally, these dependencies do not provide actionable information to
the developers reading / tools consuming the published POMs.

Optional/Provided dependencies are **not**:
* dependencies you can add to enable some supported feature
* dependencies versions that you can use to figure out CVEs or bugs
* dependencies that might be missing in existing Spring applications

In the context of Spring Framework, optional dependencies are just
libraries are Spring is compiling against for various technical reasons.
With that in mind, we are not publishing that information anymore.

See gh-23282