Convert Gradle build to use new propdeps-plugin

Replace existing 'optional' and 'provided' Spring specific build
extensions with a new Gradle propdeps-plugin. Optional and Provided
dependencies are now defined use dependency configurations.

The new plugin does not currently support the notion of optional
runtime dependencies. All optional dependencies are implicitly
part of the 'compile' scope. This is an intentional design decision
that aims to keep both the plugin and the build simple. Since optional
dependencies are non-transitive this restriction should not cause
any real problems for existing users. The only existing dependency
affected is 'commons-io' in the 'spring-beans' project, however, this
was an optional compile scope dependency in the previous Spring 3.1
release.

Both provided and optional dependencies are no longer exported from
generated eclipse .classpath files. This fixes several tests that
would previously fail when running within eclipse. The servlet-api
specific elements of ide.gradle are also no longer required.

Issue: SPR-9656, SPR-10070