AutoProve Plugin System, prop prompt improvements

[jtoman]

Adds a way to install "plugins" into the autoprover pipeline. Currently the only extension point is the property inference. We use importlib's entrypoints functionality for symbol discovery and instantiation.

• Pre-property plugins contribute AnyPropertyGenerationInput. These are sorted and concatenated with the task prompt into a cache friendly order. These plugins run concurrently with each other.
• Post-property plugins post-process the entire property list, and are run in plugin id order (I couldn't think of another sensible default ordering).

Via packaging magic, templates placed in "templates/" as a sibling to the loader class "just work" as arguments to the builders. The "host" (core) templates are accessed via "autoprover/" namespacing; there is some jinja black magic for this to work.

Sensible attempts at caching are made throughout.

Fixes a bug in autosetup where cancellation due to a sibling task failure would actually hang the whole program (at least until autosetup finished); CancellationException is a base exception, so it didn't go through the previous "except" handler, meaning we never killed "autosetup" during shutdown. In the finally block, we'd then wait for the (unkilled) process, hanging the orderly shutdown in the case of error until autosetup finished; not what we wanted.