Support context aware block plugin creation

Problem/Motivation

After #2921626: Add proper context-awareness to Layout Builder we have gathered contexts available for Layout Builder forms via $form_state->getTemporaryValue('gathered_contexts'), but for some reason (let me know if there was a good one) it is not passed into Block Plugins where it would be expected on creation.

Proposed resolution

Simply pass the result of \Drupal\layout_builder\Form\ConfigureBlockFormBase::doBuildForm's call to $this->getAvailableContext($section_storage); to $component->getPlugin() as context.

Unfortunately, this exposes two glaring flaws in the plugin ecosystem:

1. Context Mapping is built around arbitrary input and output strings, and does not rely on Context identifiers to allow context sharing easily between plugins, so every time you pass or applyContextMapping() to a new plugin you must reverse the mapping by calling $this->contextHandler()->getMatchingContexts() for the entirety of $component->getPlugin()->getContextDefinitions().
2. Multiple context providers can exist for the same context definition type, and it's not clear which one would be correct.

Here's a rewrite that effects a workaround (patch incoming):

\Drupal\layout_builder\Form\ConfigureBlockFormBase @ 165

$contexts = $this->getAvailableContexts($section_storage);
    $context_mapping = [];
    foreach ($component->getPlugin()->getContextDefinitions() as $context_name => $context_definition) {
      $matching_contexts = $this->contextHandler()
        ->getMatchingContexts($contexts, $context_definition);
      $context_mapping[$context_name] = array_pop($matching_contexts);
    }
    $this->block = $component->getPlugin($context_mapping);

    $form_state->setTemporaryValue('gathered_contexts', $contexts);

Notes:
#3018782: Remove extraneous context mapping of layout_builder.entity would be slightly helpful here, but doesn't resolve the root of the problem.

Remaining tasks

(reviews needed, tests to be written or run, documentation to be written, etc.)

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

(Major and critical issues should have a snippet that can be pulled into the release notes when a release is created that includes the fix)