Add support for CNT_IMIT TLS 1.2 GOST cipher suite

This adds support for CNT_IMIT ciphersuite from draft-smyshlyaev-tls12-gost-suites-05. It is using numbers assigned by IANA now, so it is not compatible with existing deployments till they are updated to use newly assigned numbers.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code