Introduce context bootstrap strategy in the TCF

Work done in conjunction with SPR-5243 and SPR-4588 introduced physical
package cycles in the spring-test module. The work performed in
conjunction with SPR-9924 uses reflection to resolve these physical
package cycles; however, prior to this commit the logical package
cycles still remain.

Furthermore, over time it has become apparent that the Spring
TestContext Framework (TCF) could better serve application developers
and especially third-party framework developers by providing a more
flexible mechanism for "bootstrapping" the TCF. For example, prior to
this commit, default TestExecutionListeners could only be registered by
subclassing TestContextManager (and SpringJUnit4ClassRunner if using
JUnit). Similarly, the default ContextLoader could only be set by
subclassing SpringJUnit4ClassRunner for JUnit and by copying and
modifying AbstractTestNGSpringContextTests for TestNG.

This commit addresses the aforementioned issues by introducing a
bootstrap strategy in the TestContext framework that is responsible for
determining default TestExecutionListeners and the default
ContextLoader in an extensible fashion. The new TestContextBootstrapper
SPI also provides a mechanism for supporting various types of
MergedContextConfiguration depending on the feature set of the context
loaders supported by the strategy.

The following provides an overview of the most significant changes in
this commit.

 - Introduced TestContextBootstrapper strategy SPI, BootstrapContext,
   and @BootstrapWith.

 - Introduced AbstractTestContextBootstrapper,
   DefaultTestContextBootstrapper, and WebTestContextBootstrapper
   implementations of the TestContextBootstrapper SPI and extracted
   related reflection code from ContextLoaderUtils & TestContextManager.

 - Introduced BootstrapUtils for retrieving the TestContextBootstrapper
   from @BootstrapWith, falling back to a default if @BootstrapWith is
   not present.

 - @WebAppConfiguration is now annotated with
   @BootstrapWith(WebTestContextBootstrapper.class).

 - CacheAwareContextLoaderDelegate is now an interface with a new
   DefaultCacheAwareContextLoaderDelegate implementation class.

 - Introduced closeContext(MergedContextConfiguration, HierarchyMode) in
   CacheAwareContextLoaderDelegate.

 - DefaultTestContext now uses CacheAwareContextLoaderDelegate instead
   of interacting directly with the ContextCache.

 - DefaultTestContext now delegates to a TestContextBootstrapper for
   building the MergedContextConfiguration.

 - TestContextManager now delegates to TestContextBootstrapper for
   retrieving TestExecutionListeners.

 - Deleted TestContextManager(Class, String) constructor and
   SpringJUnit4ClassRunner.getDefaultContextLoaderClassName(Class)
   method since default ContextLoader support is now implemented by
   TestContextBootstrappers.

 - Extracted ActiveProfilesUtils from ContextLoaderUtils.

 - Extracted ApplicationContextInitializerUtils from ContextLoaderUtils.

 - MetaAnnotationUtils is now a public utility class in the test.util
   package.

 - Removed restriction in @ActiveProfiles that a custom resolver cannot
   be used with the 'value' or 'profiles' attributes.

 - Introduced DefaultActiveProfilesResolver.

Issue: SPR-9955