proxy: c-frontend

[chirag-parmar]

This PR implements a C frontend for the verified proxy so that it can be used as a static library

Notes and Pointers for discussion:

1. The async event loop of chronos is exported in a single threaded manner (processVerifProxyTasks) and must be called (or polled) regularly for efficient processing of the underlying async tasks. This means any busy waiting in the main loop of the application is forbidden (which is the case independent of this PR). This is a deliberate choice, since it is easier to develop multi-threaded logic in other languages/frameworks(ex: Go and Rust) where this library would be used rather than ship the library with the async event loop running on a different thread.
2. Only the async tasks exported over the C API are tracked by the exported poll function (processVerifProxyTasks), any other async tasks dispatched independently will not be polled.
3. Some of the logic in nimbus_verified_proxy.nim is abstracted/refactored out to be reused for the C frontend (verifproxy.nim). The PR also removes the stop procs from the LightClient to allow the C frontend to manually cancel the future associated with it.
4. Since most parameters of the exported API are serialized strings (char *) docs are added to explain the encoding required for these strings.
5. Example usage of the library can be found here: nimbus_verified_proxy/libverifproxy/example.c, the instructions for compiling are added to the README within that directory.

Meta: #3101
Experimentation: chirag-parmar/nim-playground@a877be0

[tersec]

Nimbus verified proxy CI / linux-amd64 (pull_request)

 /home/runner/work/nimbus-eth1/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system.nim: In function ‘_ZN6system5resetE3varIN10verifproxy7ContextEE’:
/home/runner/work/nimbus-eth1/nimbus-eth1/vendor/nimbus-build-system/vendor/Nim/lib/system.nim:941:74: error: ‘pthread_mutex_t’ has no member named ‘abi’
  941 |       obj = default(typeof(obj))

is due to:

• error: ‘pthread_mutex_t’ has no member named ‘abi’ in refc with reset/{.exportc.} nim-lang/Nim#25205