[Boost] Add an API using WordPress actions so other plugins can interact with the cache.

Building upon the actions described in #35784 and being implemented in #35946 we will need more actions to allow third party plugins to interact with the cache.
This will allow them to delete or perform other admin tasks on the cached files for particular URLs or for different users.

These actions must be documented somewhere on the Jetpack support site eventually, but immediately as soon as they are implemented in a .md file in boost/docs/.

ref: pc9hqz-2CQ-p2

Read the actions described in #35784 and implemented in #35946, then inspect the Boost cache-related code and its existing action entry points. Define and implement the additional actions that let plugins manage cached files for URLs or users, and document them in a Markdown file under boost/docs/.

Written by the indexing model from the issue text.