Use symfony/var-exporter for encoding data in PHP cache backend

I did some benchmarking locally. Getting cached items back is almost instance when using VarExporter.

Here are some results of profiling Drupal site with cache.container implemented with PhpBackend

Here are the results:

For local Drupal 11 installation the performance boost was about 5-10% for anonymous visitors.

Relevant conversation: https://www.reddit.com/r/PHP/comments/vlyw1g/benchmarking_serialization/

Wonder if it's better to create a new cache backend.

@chi mind updating MR to point to 11.x vs 11.1.x

Also would see if answering some of the questions from https://www.drupal.org/about/core/policies/core-dependency-policies/depe... → into the issue summary could help move this along.