Introduce context bootstrap strategy in the TCF

This issue is a "work in progress".

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

 - Extracted ActiveProfilesUtils from ContextLoaderUtils.

 - Extracted ApplicationContextInitializerUtils from ContextLoaderUtils.

 - TestContextManager now delegates to TestContextBootstrapper for
   retrieving TestExecutionListeners.

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

 - Introduced DefaultActiveProfilesResolver.

 - Introduced BootstrapTestUtils.

Issue: SPR-9955