Add support for non-GitHub-hosted caching for self-hosted runners

Describe the enhancement

Right now, Action caching isn't very useful when using self-hosted GitHub Actions runners: I have runners deployed on my own hardware but the cached data is being sent to and from GitHub cache servers. The time and cost of making those transfers across the Internet and back makes caching almost entirely pointless for self-hosted runners.

Instead I'd like the ability to use the GitHub Actions caching API to cache data on my own servers nearer my runners.

What's missing:

• there's no downloadable cache server implementation that we can use to run a cache server somewhere else
• there's no documentation for how to write such a caching server even if GitHub doesn't provide one
• it doesn't look like there's any ability to control the action/toolkit cache package to make it work with a non-GitHub server. I can see there's an ACTIONS_CACHE_URL that is provided by the runner environment and points to a GitHub server, but there's no easy way to override that. Even if there were, it looks like the cache system authenticates using ACTIONS_RUNTIME_TOKEN, which also lacks documentation or the ability to override. It's also used for more than just caching, so if I did somehow override it to point to a custom caching server, it would break something else in the actions runner.

Additional information
Add any other context about the feature here.

Start by reviewing the action/toolkit cache package and how it handles ACTIONS_CACHE_URL and ACTIONS_RUNTIME_TOKEN. Define the scope across a downloadable server, cache-server protocol documentation, and configurable endpoint and authentication behavior; done should provide a usable documented path for caching on self-hosted infrastructure.