[5.x] Make parent_uri and other entry data available in static caching invalidation rules

[duncanmcclean]

This pull request makes it possible to use entry data as variables inside static caching invalidation rules, allowing you to configure slightly more advanced invalidation rules.

I've had a few sites in the past where I've wanted to invalidate URLs based on entry data. However, since there's no way for invalidation rules to be dynamic, I've had to opt for creating event listeners.

Here's a few examples where the ability to use variables in invalidation rules could come in useful:

• In one of your templates, you output a list of any child entries.
  • When you update one of those child entries, there's currently no way to invalidate the parent entry without using a wildcard or creating a custom event listener.
  • With this PR, you'd be able to simply use {parent_uri} as an invalidation rule.
• Let's say you have a Blog collection and each of those blog entries can relate to a Topic taxonomy term. Whenever you create/edit one of those blog entries, you want to invalidate the topic's "show" page.
  • Currently, when you update one of the blog entries, there's no way to invalidate the topic's show page without using a wildcard or creating a custom event listener.
  • With this PR, you'd be able to use /topics/{{ topic }}{{ slug }}{{ /topic }} as an invalidation rule.

No issue related to this one, just a small paper cut I ran into again recently.