Use tar archive instead of npm for spec tests

[mpetrunic]

Problem:
We used npm to distribute spec tests, but spec tests reached size limit of npm registry.

Solution:
Extend lodestar-spec-test-util to download spec tests archive from https://github.com/ethereum/eth2.0-spec-tests/releases/download/v1.0.0/general.tar.gz and resolve file path relative to extracted spec tests.

Method that should be extended:
https://github.com/ChainSafe/lodestar/blob/master/packages/lodestar-spec-test-util/src/single.ts#L61

My suggestion is to add spec tests version as required param and extraction location as optional (defaulting to node_modules/.cache/@chainsafe/lodestar-spec-test-utils).
Upon executing describeDirectorySpecTest, we should check whether directory <spec_tests_location>/ directory exists.
If it does resolve spec test path against it.
If it doesn't, download tar archives for given version extract them all into <spec_tests_location>/ location