Saving a block doesn't clear it's cache tags

It seems like:

1. "Normal" blocks get a block entity, Drupal\block\Entity\Block, which extends ConfigEntityBase which extends Entity.
2. Saving any Entity calls postSave() calls invalidateTagsOnSave() calls Cache::invalidateTags($this->getCacheTagsToInvalidate());
3. Context blocks, don't have a "normal" block entity, instead they have Drupal\context\Entity\Context.
4. Saving that context entity should automatically call Cache::invalidateTags() just like any other entity (but I haven't tested that).
5. Drupal\context\Plugin\ContextReaction\Blocks::execute() doesn't add the context cache tags to the block it renders, so Cache::invalidateTags($the_context_tags) won't invalidate the cached block rendering.

So I think the fix is actually just to add the context entity cache tags to the block render array we return from Blocks::execute(). We shouldn't need to explicitly call Cache::invalidateTags().

Attached is a "proof of concept" for a fix, I haven't figured out how to get access to the current context from inside the Blocks ContextReaction plugin yet; possibly need to pass it down from ContextManager? Also need to DI the entity type manager.

With this patch changing block configuration for a context block on my homepage will refresh cache for that block, without this patch changing block configuration for a context block on my homepage won't take show any effect until I manually flush cache.

Actually attach the patch.

Actual patch (was easier than I thought to get the context) - this works on my local.