tor-ptmgr: make managed PTs optional ('managed-pts' feature flag)

This adds a new default 'managed-pts' feature to tor-ptmgr to build support for managed PTs. If a managed PT is provided in the config toml or builder, a ConfigBuildError::NoCompileTimeSupport will be returned during builder validation. Closes #1334 (closed).

I tried to separate out a lot of the managed PT code into a new 'managed' module, but there's still some #[cfg(feature=...)] soup in the main lib code. I don't think it's too bad, but I can do more refactoring to reduce this if needed. I didn't want to change the behaviour too much so as to not introduce bugs.