Feat/no threads feat for zk sdk

[leonardkulms]

Problem

Currently, the spawning of threads makes it impossible to use the zk sdk in a wasm environment.
At the same time, per default it only uses one thread (and is quite fast still) anyway.

I would like to use the zk-sdk in a wasm project without having to fork it or (as I currently have) to copy a big part of the DiscreteLog functionality and replace the little part where it spawns a thread in a similar way as this PR does.

Summary of Changes

• adds a feature called enable-threaded that is enabled by default
  

• consequently does what it has done before, unless enable-threaded is disabled: it will do the same but without spawning a thread for it.
  

• adds a test that can also be compiled with feature flag as e.g.
  

cargo test --package solana-zk-token-sdk --no-default-features

[anza-team]

Thank you for the contribution! This repository is no longer in use. Please re-open this pull request in the agave repo: https://github.com/anza-xyz/agave