Automatically download new compiler versions in contract-verifier

[popzxc]

Right now, all the compiler versions are included into the contract-verifier image.
It means that each time a new compiler version is released, it has to be manually added there.

What we want is to be able to download compilers on demand without having to rebuild docker images.
How it should work:

• Compiler versions are checked periodically via GH releases.
• These versions are marked as available.
• If a compilation requests happens for a compiler version that is currently absent on the pod, it's downloaded from GH releases (e.g. it's lazy downloading; there is no need to download everything at start) and then persisted locally.

Considerations:

• The implementation should work correctly even if we support parallel processing of compilation requests (e.g. if we have 2 requests to compile a contract with currently absent compiler, we should download it once, not twice).
• The implementation should handle network errors (e.g. if download request fails, we must retry it).
• There should be no deadlocks (e.g. if we repeatedly can't download a compiler, it should not stop processing of other requests).