* pr/7510:
  Remove redundant toString() invocation

Closes gh-7510

Downgrade `maven-assembly-plugin` from 3.0 to 2.6 since the later
version contains a bug that means `recompressZippedFiles` is not
respected.

See gh-7513

* gh-7263-2:
  Add custom LayoutFactory sample
  Support custom fat jar layouts
  Rework Repacakger timeout code

Add a sample that shows how a custom LayoutFactory can be used with
both Maven and Gradle.

Closes gh-7263

Allow support for custom Lyout implementations with both the Maven
and Gradle plugin. Implementations of `LayoutFactory` can now be
specified to allow customization of the layout. In addition a
layout may now implement `CustomLoaderLayout` if it wishes to
write custom loader classes.

See gh-7263

Pull up common timeout code into Repackager and remove the need for
custom subclasses.

See gh-7263

Closes gh-6763

Checking for the presence of a `ValidationProvider` resource is
unfortunately not enough to trigger the validation auto-configuration.

If `hibernate-validator` is added on the classpath without a `javax.el`
implementation, the former will blow up on startup. So far this was a
pilot error so we didn't have to care about it. Now that we have an
auto-configuration that may lead to this error, we need to be extra
careful and check that scenario ourselves.

This commit adds an extra condition that runs as late as possible and
attempt to actually initialize a `Validator`. If that fails for whatever
reason, the auto-configuration will now back off.

Several additional tests have been added to exercise this scenario.

Closes gh-6228

See gh-7226

Closes gh-7496

Cast runtime object retrieved reflectively to their public interface
counterpart.

See gh-7226

Closes gh-7489

See gh-6995

* pr/7494:
  Add ValidationAutoConfiguration to spring.factories

Closes gh-7494